That sounds pretty nasty.

w.r.t. garrisons, i would say that for cities only allow the up to the number of free upkeep units (plus one general/governor). For castles, say, a maximum of 4 units on top of generals.

w.r.t. number of stacks, how about limiting you to one mobile stack for each faction you are at war with? Obviously you can bolster your garrisons when youre at war. So if youre at peace you wont have any standing army at all and will have to raise it from scratch if you go to war. Once the war is over disband all of your troops except your bare minimum garrisons.

Of course, this means that you will probably have massive levels of cash to play with each turn, since you have minimal troop maintenance costs, so you will probably be way out ahead of the opposition in terms of tech.