I love your idea, OP!

Another thing I thought about was changing how province conversions work. Barring a few extreme scenarios like the Reformation, changing the religion of an entire province (even if we go by the logic of it only being the religion of the majority population) should take far, far longer than your average 1-2 years. We see in the real world that it takes a very long time for a new religion to take root and it's virtually impossible to eliminate all adherents of the previous one. It's immersion-breaking to be able to conquer most of Arabia and flip it to Catholicism in the span of 5-7 years.

So I thought of a slightly different mechanism - making it a process kind of like westernization. You can turn it on at will in any/all provinces (no random missionary cap and you can pause it) and while it's on it slowly ticks to being the majority religion, but you get the increased unrest and whatnot. Pro would be that you could mass-convert your empire without the arbitrary 1-4 (average) missionary cap (so I can convert this region but not that one? Why?), but con would be that I'd increase the time needed dramatically. Say 20 years as an average, with ideas and policies increasing or decreasing this.

I think it's more realistic and it gives you the option of converting a large region comparatively quickly if you can weather the unrest (simulating, for example how the Ottomans did most of the conversion in the few decades after conquering the Balkans, not province by province, but in a large area, and not in a year or two, but over several decades).

I don't think this would actually be that difficult to implement code-wise, but it's probably game-breaking in some way I haven't thought about yet.