There is no quick way.

In my experience, Inquisitor realms of any consequence grow only with "Apocalyptic" Inquisition game rule. If you don't like the outcome, you might want to adjust it for the next hunter playthrough.

Not judging, but from RP perspective, I don't really understand your motivation at this point. If it's just you and other Inquisitors, the Evil has already won, rejoice.

Honestly it made the game way to easy, and messed up some of the things i wanted to do. For example i made my leader a immortal mummy. But i guess i gotta scrap it.

to be honest i remember one time when one of my player heirs was the ♥♥♥♥♥♥♥♥♥♥♥♥♥ head of the inquisition, should i could just send all inquisition members an vassalization offer and they would accept it instantly because the modifier was something like 1000+ or so, it was freaking wild

Would not work in OP's case. He specifically mentioned them being empire titles.