Like I cannot fucking figure it out and since I'm BANNED ON REDDIT AGAIN I can't post on the one place where someone might actually answer this other than here or I guess the steam forums but fuck the steam forums.
Anyway, I have never, ever, in like hundreds of hours of gameplay, taken territory and not had a secession shortly after that I've had to crush. The turmoil in conquered territory is always >50% and it's basically impossible to build enough to meaningfully affect standard of living to reduce radicals before the secession triggers, especially because of the massive construction penalties from the turmoil.
Like I do not know what I, the player, am supposed to actively do aside from wait for the inevitable secession and crush it. Then it never happens again and the turmoil goes away.
Like am I supposed to just never conquer anything until I have 10000 construction points and can turn the conquered territory into a utopia in a few months? Because the timeline to secession is IMPOSSIBLY short.
Agreed, I don't think you really have enough time to stop it. The trigger is having over 50% turmoil and 33% population belonging to the ethnicity in question. I think going multiculturalism helps reduce turmoil because there'll be no discrimination against the newly-conquered pops, but it probably won't make that much of a difference. I don't remember if having cops or using the decree for suppression reduces turmoil itself or just the effects, but those are another option.