It's just classical trolling in a world where people no longer know how to deal with trolls, which is quite simple: don't feed them. Flat earthers by contrast are feasting like no troll ever before.
I would probably agree with you based on my participation in these groups (have moderated them, don't ask why, it's just a weird/funny hobby to me) that it is much lower. The 5-10% number is the estimation I've received from other moderators in this space (if anyone is also in this space feel free to chime in, I find it fascinating). However, it's hard to estimate, because frequently genuine users get trolled/harassed into oblivion and end up leaving because of it. So the longer a user is around, the less likely (IMO) that they are a genuine believer and probably a troll. There are prolific unicorn "believer" users that drive a lot of conversation but are a very small minority.
As far as the number of people out in the wild who are flat earth believers or flat earth curious, the amount of views/interaction from FE "influencers" (who I don't believe are actually believers) would suggest the actual number is surprisingly high.
And you're absolutely spot on about what drives engagement in these types of groups - often the people that are there to freak out at flat earthers are themselves not the most intellectually curious or rigorous people, and are just there to laugh at the people they know for a fact are "dumber" than them. Pushing back at that psychological dynamic ends up with some pretty funny troll-worthy content, at least IMO.
I found this out when the 10-year-old son attempted to lecture me on how I should "do my research"--by which he meant, study the Bible.
The idea that everything was made up of 4 elements (or a rather a combination of those) also assumed a round earth. Early things are heaviest and sink to the bottom, water is lighter than earth, air lighter than water and fire is lighter than air (that’s why the stars, made up of fire, are at the very top)
The church never disputed the earth being round. They were pretty adamant about it being the center of the cosmos though, with the sun orbiting it.