Ive heard the other slots unlock by getting a % of the population on your home or distant lands high enough, that unlocks another slot. Like 70 or 80%. I have yet to come close to that number, or see any stream/video hit that number and unlock them. Frankly, unless you blitz religion faster than the AI, or they ignore it, getting that high of a % feels like a waste of resources for what you get. The movement rules + needing to convert twice if they have a religion, makes it very time consuming. Its essentially an uphill battle. If you had a way of defending your cities other than just reconverting them, then maybe it would be better. but then we would be back where civ 6 was.
At least you can convert holy cities now which blocked that % requirement.(if true) Not to mention making getting relics easier.
Unless you plan on running those 2 policies that require your cities to follow your religion, you are better off not bothering converting your own cities. Your religions bonuses apply to foreign cities, not yours. If you run the tolerant policies for the crisis, you can also ignore your own cities.