Nebulous
Legend
One issue I have with the module is that there is almost no interaction with cult leaders until it is time to fight them. They are given interesting personalities and it seems a shame to let them go to waste.
This is a big issue in many adventures, no less in the author's former Phandelver module. There, I had the Black Spider foreshadowed as the main villain nearly every session, his name was dropped all the time, and then in the final Wave Echo Cave scenario had head automaton headcrabs that spoke with his voice so that the PCs could interact with him (and make a deal) long before they killed him. And seriously, he died in 1 round, if I hadn't done all of that foreshadowing it would have been the lamest villain ever. And THEN he got zombied by a beholder mage, so even then his character was still around.
So yes, plumping out the high priests of the cults is a good idea.