I may have missed something at some point, but how is it that my religion, Nubian polytheism, came to be led by my neighbour? It was named after my people, but it's their colour and they made all tenet decisions. I'm wondering if it's a bug or if maybe I missed a prompt with it somewhere.

Also relating to faith, is there a point to it after you unlock all four tiers of tenets? Is it just then about getting higher faith output to spread it further and gain more bonuses from existing tenets?

The religious leader (not the founder) get to chose tenets

The religious leader is the player with that state havign the most holy sites iirc (note that some Wonders count as Holy Site)

Faith only use is to spread your faith, some tenets can give you bonuses depending of the number of followers/territories you are correct, so this is good to spread more

Then if you dont have bonuses depending of this, it mostly depends wether you want to convert people to your faith, which can give you grievances if they have a different state religion

At that point you should probably invest in other things than faith