2 things to look into to solve your Gordian knot:
- transfer vassalage: this'll make a lot of people happy. However, you'll piss them off once you disband the empire.
- it's possible to take the primary title from infidels. But I don't think orthodoxy is considered as a different religion for Catholics. Worth checking though. Another reason why you shouldn't have changed religion
Or you could make all your secondary heirs bishop, problem solved.