It must be remembered that whatever gets built is dependent on the industrial buildup and in turn on investment and labor policies from the government and a few powerful private manufacturers. Upgrading existing stocks may not necessarily be doable at the same time as producing new gear. That's why even though upgrading the short 37mm guns and 75mm guns beyond OTL decisions was certainly technically feasible, it won't necessarily improve the overall situation. OTL, spamming more 105mm artillery was made impossible by committments to the AA and antitank gun programs, which were an arguably more pressing matter when France OTL could already meet its minimum artillery needs.
AA is certainly the field of artillery that needed the most extra effort, both in terms of the guns themselves but also more importantly the additional elements (fire control). That sector seems to have been plagued by perfectionism which delayed the introduction of the 13.2, 25, 37 and 90mm designs even though the French had a severe shortage of guns in these categories in the first place. The modernisation programs were hit hardest by budget cuts before 1935 and the post 35 effort was a bit late (bulk of the spending happens after 1938).
A lot of the systems France fielded in 1940 had been selected before 1935 and the exigencies of mass production require a sufficient lead time, so these systems can not be cut out too quickly after 1935, and our material focus should be on the post 1935 programs and the distribution of follow-up orders.
Doctrine and materiel are also interelated because while doctrine can help specify what is needed or is structured with what exists in mind, at the end of the day your forces still need to have the materiel on hand to verify or invalidate the doctrinal concepts and to train sufficiently to actually apply that doctrine. There were recurring themes in OTL France of training grounds or special schools being closed as part of the deflationary policies before 1936, of large field exercises or reforms not taking place in peacetime due to lack of equipment or geopolitical crises (Rhineland, Anschluss, Sudetenland). A lot of the equipment was only delivered in after 1938 or even during the mobilization period in 1939-40, and before that, was so rare that it had to be used parcimoniously to not wear too quickly.
Training, especially in large maneuvers, was quite neglected in spite of the concerns of many mid/high level officers, which is an oddity for an army so dependent on the good combined arms warfare of enormous conscripted armies.
Of course, you sometimes have to compromise with what you have and Germany demonstrated a willigness to adapt even with limited equipment before 1940, but overall it would have been important for France to have a smoother rearmament with greater production in 1935-39 even if the eventual ceiling was no higher than that expected in 1940-41 OTL.
I'm more versed with the tank situation, so I will focus on that more specific topic from now on. The good thing is that I have found an archived document from the General Tank Inspector (General Keller) from 1941 precisely looking at the lessons learned from the battle of 1940, plus some letters or comments from previous actors in the field of French armor.
Doctrinally, there were some good things in the Instruction for Large Units of 1936 (IGU 1936 in French), subsequent 1937 amendments and the 1938 Instruction for the DCR type Armored Divisions.
The core issues in doctrine or execution can be summed up as such (notwithstanding all the remarks on maintenance, logistics, organisations and comms which inevitably arise with battle experience):
- the IGU 1936 was built largely around the thinking that antitank guns were the same threat to the tank as the machinegun to the infantry. This was flawed in that antitank guns were slow firing, less maneuverable and harder to hide, and that the French tanks after 1933 had precisely been armored to provide a good degree of protection at medium ranges, which is something the IGU itself admits. Combined with a fear of losses and risk, this largely explains why tanks were kept so subordinated to the infantry and artillery instead of being able to act independently (if necessary to take out the AT guns as well) or as the vanguard with support from the other two branches. Of course, this was secretely influenced by the bias against the idea of tanks as its own separate branch like in Germany, the UK or US.
- not enough focus on massed employment of tanks. The 1938 instruction for the DCR does emphasize massed employment but the DCRs were too inexperienced anyway. The IGU however allocated only one or at best two companies of tanks (13 and 26 tanks respectively) to a battalion-level infantry attack, far too little. Too many tanks were part of general reserve independent battalions, not enough as part of large tank units (DCR or DLM). This was not helped by the complete inexperience of many infantry commanders who, unlike in WW1, felt they knew better than tank commanders and misused those independent battalions by hoarding them after an attack (preventing maintenance or regroupment) and spreading them very thinly. The obsession on continuous fronts which was a more general issue in the army contributed to tanks being spread extremely thinly. It was more important to be strong in a few places than weak everywhere. Overall, the doctrine simply required too many tanks compared to what the French had in 1940 to do everything. It might have been doable in 1941 with production now being massive, but in 1940 or before it wasn't. Keller felt that with the numbers France had, they needed to gather around 200 tanks to be really effective against the Germans who massed tanks.
- poor infantry/tank cooperation. This is correlated with the closure of the Tank/Infantry school in the 30s and the lack of tanks in peacetime, such that the infantry rarely saw anything more modern than a Renault FT. Infantry commanders hoarded tank units, asked too much of them (not enough time for maintenance), sent them on dangerous or useless missions where they were slowly attrited... The counterattacks were often rushed and done piecemeal. For example one DCR attacked with 50 tanks only when 48 hours later it could have assembled 168 a force of tanks. Time is of the essence but what can you do with such meager means other than feeding limited ressources to the enemy...
- Late constitution of the DCRs. The DLMs were more or less okay, but the first two DCRs were only formed during the Winter of 1939/40 and many of their constitutive elements only trained together very shortly before May 1940 and many elements were still understrength. This is in spite of the fact that the manuals and training regimes were ready for a constitution in peacetime in May 1939 or so. Overall, the DCRs needed an additional 3 months of training that can only be obtained with sufficient production (or a willigness to strip normal infantry units and independent tank units) early. Even then, the DCRs were often stripped of important elements during the battle.
- Lack of second crews to relieve the exhausted crews, like done in the UK and Germany. Fatigue after the first few days was a big factor in French problems during May/June 1940.
- Not enough focus on specialization. AA and Engineers were hit particularly hard here as the former required specialist personnel and some of the latter had to specialize for use in armored divisions.
Production wise, you may be surprised that the actual costs of tank programs recommended by high level tank officers were not actually that high compared to the sheer expenditure after 1936. Usually around 2 billion francs, or the cost of a new battleship. The Maginot Line itself was not the biggest obstacle to spending on a proper maneuver army, the general political/fiscal consensus until 1936 was, and in any case everyone involved always intended BOTH the fortifications and the army to complement each other.
One big issue was that no large orders over a long period were made for most of the 1935-40 period to get the manufacturers out of their sleep and organise for true mass production, which would kick the French hard before things were finally somewhat sorted out by late 1939.
So, what about the tanks themselves?
By 1935, most of the key components of the 1940 force were adopted or were about to be:
- Char B1 Battle Tank
- Somua S35 (Cavalry tank, but de facto a Battle Tank)
- Infantry light tanks (Renault, Hotchkiss and FCM)
With limited orders of the Char D2 Battle Tank, and the orders for previous programs to be completed more or less quickly. Battle Tanks were most suitable for armored divisions due to being fitted with a radio and dedicated operator from day one, and due to their better armament and crossing capability.
Other than the wartime Keller program to replace all tank classes in 1941-42, the only tank program for common vehicles was the G1 Battle Tank. Originally, it was conceived as an Infantry "S35" or "modern D2", so a cheap, fast battle tank which was simple to operate by conscripts, to complement the B1 which was complex enough to truly require professional personnel, was too slow for the motorized infantry divisions that France wanted to make, was expensive and not quick enough to build. It also was to be under 20 tonnes as beyond that you see a sudden drop in logistical convenience (specialized wagons needed, fewer compatible bridges... For reference, 1930s France had 3 main logistical categories: under 13 tonnes, under 20 tonnes and under 35 tonnes, the latter being relaxed to 45 tonnes in 1940 owing to improvements in logistical infrastructure and the need for greater performance).
In practice, it started in 1936 as a "modern B1" with the same dual armament. Finally, in late 1937 it was adjusted to have a single 75mm gun in a 2 or 3-man turret. The unfortunate consequence of this is that the proposed designs had been designed with the "S35" or "B1" configuration in mind, such that the hull form factor was weight-inefficient when going to the 75mm turret (huge hulls designed for a hull gun) and the engines were a little weak. The constant changes also demoralized manufacturers who put the projects on the backburner for much of 1937 and 38, such that nothing was expected other than the Renault G1 in mayyyybe 1941.
Considering the relative importance of the Battle Tank compared to the Infantry Light Tank in every single document after 1935 and in armored divisions, it would have been best to orient limited funds and industrial means to that category even at the cost of the total number of tanks both classes combined. The situation for the Infantry Light Tanks could have been made better, but overall they never posed the industrial/fiscal challenge than their heavier brothers did. IMO, the G1 program was the centerpiece of the entire problem and is where you need to make the biggest departure from OTL.
The time to make a "better S35" or "B1" had passed. Even if they stuck to either configuration, the winner(s) would inevitably had entered production well after the alternatives: simply focusing on the B1 and S35 series. On the other hand the configuration they eventually arrived at in 1938 (75mm turret, non-envelopping tracks, compact hull with rear-drive) was very progressive. IMO, even if the French had simply started that program in late 1937 around this configuration, so no later than OTL, it would at least have benefitted from the proposed designs being tailored to it from the very start. I also think that this configuration was much better suited to meet the future battle tank program for 1941-42 than either the B40 (compromised by its Char B roots so heavier than it needed to be for a 75mm turret) or its competitors; all that would have been needed is updating the program to account for higher mobility and 70-80mm of armor.
In the meantime, the cheap battle tank role could be taken by the stopgap D2 (at least order 3 battalions/150 tanks from the start, consistent with the original 1934 plan and the order for 160 D1s before) and the S35 (the production potential of which was always quite underexploited before 1940). Fixing some issues in the form of the S40 would have been even easier to do if the tank catches such a bigger interest and if Somua does not waste time in the G1 program.
The B1, meanwhile, would benefit from the lack of competition from the G1. It would be possible to bring the peacetime orders (300 or so) closer together in time and to invest in the manufacturers which were behind schedule for certain parts to get to the sustained 10/month production rate mentioned in 1936. This alone would accelerate completion of B1 battalions so much earlier that it would be possible to experiment with DCRs early with B1, D2 and S35 battalions. The B1 Ter, which was originally (1934-35) intended to be the third batch of B1s just after the B1 Bis, and thus intended to be built in 1936, could meet most objective of the "modern B1" part of the OTL G1 program by complely overhauling many assemblies (electrical system, cooling system, hull gun mount, lubrication system, transmission) and simplifying production, maintenance and use. What it needed was more focus and an investment necessary for 3 new-built prototypes, as using the prototype built from an older B1 was simply too slow.