Hello Shortround
on horse pulled artillery, yes, I understand well what you are saying. In the small farm where my father was born was a horse. We visited the farm every summer to help my uncle and my cousins in hay making etc so I saw something the horse, happened to be a "war veteran", did in agricultural surroundings. And on the effects of load, while I did my compulsory military service I learned, that while I could run, even uphill while carrying four 10kg (22lb) A/T mines, I could not ran very far with that load, while carrying 2 A/T mines I could ran clearly longer distance but still not very far but after a while it didn't do much difference on my running if I had one A/T mine on my back under my combat harness or not .
I wrote mostly on organic field artillery of infantry formations and not on independent heavy batteries or battalions just because of the organic FA units were supposed to be able to be capable to move as fast as their parent units.
BTW according to a WWII action photo in my field gun book, Finnish Army used 8 horses team to pull 155 H/17 aka Canon de 155 C, mle 1917 Schneider. and the battery was moving along a forest track while changing firing positions
And back to topic, the best light FA gun, maybe Soviet M-30, 25pdr or Swedish/Finnish 105H/37. I don't have now time to check the US 105mm, it might well be a good candidate, too.
Juha