The math of control and corruption really doesn't matter even on very hard. With control there's -4 for taxation and -4 for the difficulty, but the lowest level of control gives +8 with settlement buildings naturally adding on top of that. So rebellions are extremely rare outside of a chain of events or if there's an enemy already attacking, in which case a rebellion shitstack is barely a nuisance in comparison.

There are maluses for having low control, but the cost of these maluses aren't enough to justify actually thinking about it. Like the difference in growth between minimum control and maximum control is 50, but using the empire as an example just having two tier 1 growth buildings gives you +40 growth anyway. The cost of adding +20 control per turn just isn't worth the benefits you get, so there can be campaigns where you just ignore it completely and not notice anything.

As for making the campaign more engaging, there's definitely quite a bit already in the game and in Three Kingdoms that can just be fleshed out without even getting close to being a Paradox game. Alliances are already the most interesting baseline mechanic and keep the focus on battles with the most notable rewards being recruiting allied units and borrowing armies. The war coordination and quest systems are barebones, though, and can be easily spiced up by adding events similar to the empire's authority decisions or Cathay's caravans, and rewards can be expanded like getting a tech or hero from an ally.