I’ve promoted a wild idea a couple of times. I know it’s not super popular, but I actually think it would be a good solution to this within the civ framework. What I’ve suggested is that when you train a military unit, it occupies a population point from the city. Not just decreases population by one (as Ottoman Jannisary did in … was that in 5 or 6?), but becomes an assignment for the citizen, similar to how they can be assigned to work a tile or work as a specialist in a building. This would open up for several options that I think would be interesting, such as, a) drafting units in times of crisis, b) assigning units back as workers in times of peace, and c) and easy way to include a sort of supply chain for your armies, I.e. the soldiers consume food so you can’t just build an infinitely large army.