I have been sleeping on the Recognition of Talent tradition

[Razorblade]

Pardoning a crime grants a permanent Strong Hook on whoever you pardon. Simply convert to some Righteous/Fundamentalist hostile Faith, send out a Conversion Demand to all of your vassals, have them deny it for a free Crime, then Pardon them and have them in your pocket until they die.

They can't rebel and they must accept your every diplomatic request, all without ever having to fight a war or put in any effort whatsoever. Absurdly overpowered.