How about drastically reducing the recruit production speed while a region has too many militias?
If many people(Population) sent out as guards, then they not farming or mining. So the production will suffer.
Sound fine to me.
Though I hope we fix that Population issue first. As many peasants(Population) unable recover for quite sometime. Example, look at Alebad city, the site of battles by many fallen realms resided there: Alebad realm, Minas Thalion and now Halcyon. I see their Production level not recover fast.