Wait, just trying to get clarify, you're using a 40-width infantry division as a garrison division? I would suggest putting together a much smaller (12-width or so) cavalry division with some form of MP and if you're willing to do a bit more repairing, civilian oversight so that compliance can build up, which is one of the best ways to deal with resistance long term. (And if you're worried about collab govs with LR, you aren't forced to make them. You can keep them at 100% compliance under your direct rule.) This should save you quite a bit of manpower for garrisons and also help keep resistance down in general long-term. While the cavalry+MP division will most likely have less suppression, you should get more 'bang for your buck', as you can get up to 3 suppression points per unit with full MP (otherwise it's 2, which is still the highest without using tanks (2.5), which could be a little wasteful depending on your situation).
Note: I generally don't do much with garrisons, so I don't know a ton about it, and I also am used to using smaller divisions, so I'm not used to the whole 'get 20 battalions worth of x and 10 battalions of y in z division'.