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.
Avoiding secessions is very hard. As others have mentioned, multiculturalism and low infamy help. As will religious secularism, high welfare pay, and public healthcare, to avoid more discrimination and rapidly increase quality of life. You're also right that buildings will help, but often not soon enough. Consider subsidising local industries to avoid unemployment.
In my experience, the most effective tool to avoid secession is using authority for local edicts - violent suppression, social mobility and emergency welfare especially. With those, I have seen the secession progress bar reverse course.
Tldr spend a lot of time and money and effort in it, or just post soldiers nearby.