>Forced conversions were pretty rare for Muslims (that's just not a thing they do as a rule, two arguments: the Quran says not to do so, and non Muslim can be a source of cash with special taxes).

You'll see some posts on /r/askhistorians about this too. But one other kind of obvious reason that the western idea of mass forced conversion, or conversion by the sword, just doesn't pan out is that often Muslims were a small minority in an area. You can't just go mass convert everyone when they have a 100 to 1 advantage over you. So it rarely happened, but there were governmental and institutional advantages to converting. You could get better patronage. There were specific additional taxes for non believers. You had a right to participate in conquest and gain land, etc. So there were lots of good practical reasons to convert.

This distrusts, and some earlier Muslims people retaining benefits only to their own class, and denying them to the new converts, manifested in the Shi'a/Sunni split. A lot of the later converting groups felt that they weren't getting their fare share of opportunities and saw Ali as willing to address that. It's not the total explanation, but part of how the split developed.