But you're not adding more ways to spend faith. You're taking the existing ways to spend faith and forcing the player to take Piety policies in order to access them. Add more ways to spend faith to the list of reformation policies, by all means, as long as they're interestingly different and don't just make faith into a duplicate of gold. But don't take away the existing founder and follower beliefs! Honestly, the Monastery solution (creating a new faith-purchase building that is in the Piety tree rather than requiring a belief slot) sounds much more promising. That is a real extra, taking up a policy slot, rather than a belief slot.