I wanted to ask about leader flavour. If the leader has no assigned one, is there some default one set for him/her or the game crashes in agony?
Some experience: I am playing with pantheon requiring 20 Faith (+2 per each founded) and each pantheon has providing faith. Religions are found by T80 (normal, immortal). First one is usually about T60. (in unmodded game, first religions were also about T60, but last ones were usually about T100).
I have linked Religious buildings to Enchancer belief. Main reason was to let non-religion civs using faith before GP. It is actually funny to live between strong religions so your cities have multi-cultured faith buildings.
In opposite to Pantheon, I made Founder and Follower beliefs not granting any faith (so there is a need to fuel religion in other ways). What was a result? Rather than one superior religion (Ethiopia + Pilgrimage <3), there are usually 4 strong ones and 3 rather forgotten. (depends how religion are distributed in regions).
I was trying to standardise things (pantheon - faith; follower/founder - various bonuses; enchancer - bonus for spreading religion and building).
What may be interesting to you is Reformation belief (mainly reason I wrote this, because I haven't found anything about Reformation in OP). I am actually using a dummy policy to trigger it (in my case, the trigger is Industrial Era, but I think you can make it more complex with all those systems you are using and create).
What is the main advantage? Policy granting Reformation is useless for civs, which haven't found religion. I think, making Piety viable for every civ is crucial. I actually don't mind going Piety tree in my games and fail to found religion (I got Happiness, faster plot acquiring and it helps in buying faith buildings, which are quite important source of Happiness and Culture in medieval era for me). On the other hand, I nerfed Tradition hard so it may be the cause.