Heresies deffinitely have a problem. There seem to be no reason implemented for them other than "oh I read about (say) Bogomilism in a book, sounds cool, I am in". Or "I am too stressed so ima gonna convert to Paulicianism". It's a choince on the same level with "I'll have an extra bottle of wine", which is ignorant.
It is good to see Bogomilism even existant in a game, but at least there should be an effort made for triggers. This one could for example have more and more chances with raising crown/tribal authority. Something else which was vital to Bogomilists (and vital to the whole Orthodox church itself, and pretty similar in the Catholicism) was the Patriarch's and, respectively, Pope, actual power and authority.
This authority doesn't even exist ingame and it's like half of the actual feudal world missing from the game. It should be a parameter in itself, similar to the crown authority, which should have actions and reasons for the Patriarchy itself. There should be Metropolitans, there should be anointing, there should be excomunicating with serious repercursions, and not "ah I don't like you in the quantity of -25" which is lolable.
There should be clerical decisions about fasting, there should be religious celebrations interfering with organizing of cities and villages, with agriculture organized around them etc.
There should be trading and priviledges, with borders actually meaning something.
There should be serfs and burghers.
And then yes, for example Bogomilism would have enough triggers. It could spread like the religion in eu4, if a heretic army ocupies a province, it is converted.
Not this joke where Orthodox in, say, southern Italy cannot convert anymore, say, some Catholics because somewhere in the steppes of Caucasus a pagan warlord has defeated a duke's holy war. Not meaning such things shouldn't be a thing, it seems reasonable. But their impact is ridiculous. Within 10 years I singlehandledly vanished Orthodoxy's fervor by defeating 4 holy wars of some dukes. It's absurd.
Holy wars themselves shouldn't be something which you can pull out of one's back every other month, just because you succesfully went to the church. There should be an effort, a holy war for a duchy should be one monarch's crowning pious achievement for a lifetime of religious work, not every two months effort of somebody who wants some extra tax and levy, for that there's claiming.
So, yea..