You should also keep the Theology character focus active for the additional monthly piety.
This cuts the Piety cost of creating a new faith in half, which makes it far more realistically attainable. If that's you, first pick up the Prophet perk from the Theologian tree in the Learning lifestyle. The thing is, this costs a lot of Piety-more than you can realistically expect to earn in a single character’s lifetime unless you're really trying. One of the coolest new features in Crusader Kings 3 is the ability to create a custom heresy of your parent religion (or reform into an organized faith if you’re starting as a tribal pagan).
