Yeah, not being able to spread your religion means you lose out on votes (and the potential utility of a good Founder as opposed to basically being locked to Hero Worship) and any other follower beliefs you could have for your own religion. And Reformation is a pain.
With a Pagoda strat, I assume you could have spread your religion and just let another civ convert you with Missionaries/Pressure, but if you're killing everybody before they have the chance to convert you, then you have to spend the Faith to first convert your cities and then spend Faith to convert your cities to another Religion, so that's difficult/inefficient. I didn't think that you'd be killing everybody ASAP, that there would be time for another civ to convert you before you kill them, which is why I proposed Pagodas.
Syncretism is hard to use efficiently, because you give up on per-Follower beliefs in your Secondaries for both the sake of your Capital and your Founder choice is limited. There are also times when other Enhancers can just be more yield-heavy with less work. But it is interesting so thanks for sharing your game!
I find it a little odd how you say your Capital is 100% Syncretism but your cities have all been converted to Catholicism? Surely all that Pressure will create at least 1 follower?
There is an absolute ton of pressure in my capital. Around 360 pressure for Catholicism in my capital compared to 30something for my Syncretism religion. So I am pretty constantly using Inquisitors to keep my capital at high Syncretism faith. But the huge amount of faith generated from Syncretism basically pays for the cost of inquisitors so it isn't too bad. As my capital gets significantly past 30 pop (the max needed for follower beliefs) it won't be so bad, but we're only at 31 right now so it'll be a while before I can really get past having to constantly deconvert my holy city.