I realized after you capture all the bases and towns, military becomes very hard to find. which in turn makes it difficult to get to heat level 5. i wish there was a mod that would increase the number of enemy spawns.
Just try the “realistic just cause 3” and everybody will fight you, including rebels and you will be really easily to heat 5 (steal a car/kill a goat/ …). (rebels will even call reinforcements to fight you and the military).
If you want military, just re-oppress a small town and run away. They’ll follow you wherever you’ll go (no chance loosing them). Also, there will be much more tanks, soldiers, … than before and reinforcement will come really faster with this mod.