Regarding taxing Religions at individual levels. At first I was interested in the idea. However if it is at the realm level, all it becomes is another tool to promote national religions by taxing all others at max level. Again an interesting mechanic, but one that requires a shift in player mentality first.
why not? rulers can promote national religions, but dukes or even ordinary lords can make additional effort to oppose it. is it not good opportunity for development of interesting grievances? hostile religions can always be banned, so taxes could be mid-solution that allow more nuances of possible conflicts.
if all agree about single religion, all will promote it with or without taxes. however, if there would be disagreement, mechanics would fuel conflicts.
i always believe more power is good if there is a chance to oppose such power. on contrary, removing power from most of positions can sterilize conflicts.
