Different cultures and rebellions are part of the challenge. If someone doesn't want to deal with that, then don't expand your kingdom so fast. Conquer a province, make sure its rebellion number is low, and then proceed to the next.
I do agree that vassalage was not used very well in KoH1. Crusader Kings 3 has a good vassal system. If we want to have a vassal system, there needs to be a goodly penalty for spinning off a part of your kingdom. Not just for convenience sake.
I know you can't control a vassal province. So you can't recruit, control income, etc. But that seems only a minor inconvenience as, by the time your kingdom is so big you need to spin off provinces, you don't really need to control them. You have enough income/resources.
No, if we are going to do this, there needs to be an appropriate penalty.