I think the issue of assimilation needs to be covered better in MMtG than it has been in previous EU3 mods and vanilla.
In these, one can assimilate a province culturally and religiously, far too quickly.
Historically, assimilation was very rare in this game's timeframe. Pretty much the only major European assimilations were the Christianization of Spain, the Germanicization of the Wends, the Russian colonization of the steppes, and in Southeastern Europe certain migrations (the Romanians, Slovaks, Albanians, Sudeten Germans, etc) and conversions (the Pomaks, Bosniaks). There was no Lithuanization of Belarus, nor christianization of Northern Africa, etc. which often happen in EU3.
Instead of assuming that assimilation is common and easily modeled by one model, working from the top down so to say, it would be better to analyze each of these and by reviewing the factors and causes for these migrations/assimilations, develop a system that can accommodate these and similar changes without allowing a culture or religion switch in five years.
I hope you can model this complex phenomenon well.
Really looking forward to playing this game, no matter what you do