An airbrush is an over-rated luxury that is nice to have, for some jobs, but not a neccessity.
OK, I use one nowadays, and have, off and on, for around 30+ years, but I still use a traditional paint brush for around 50% of my work. The airbrush is handy for large areas of clear coat, or sometimes a one-colour finish, and certainly good for mottle camouflage, but it is certainly not a 'must have as it'll make me a better modeller' tool.
Anyone who can really master brush painting, learns to understand paint and how it works, and will find that, in many cases, a paint brush is just as good as the squirty thing and, in some cases, can be a darned site more convenient. espaecially when it comes to having to mask awkward areas for fear of fine misting over-spray, where this would not be required with a hairy stick.
You're dong fine with what you've got, and an airbrush will not make things quicker. Quite the revrse, infact, because of extra masking, paint preparation, testing, cleaning and so on.
It's been said before, and I make no apologies for repeating the mantra - modelling is not a race!
One of the most important requirements of the hobby, for those who wish to continue to improve and excel, is patience. Rushing or impatience does not improve or hone the skills, it just leads to lower standards, frustration and, ultimately, despair.