How to Get Rid of Isolationism
You have to have an interest group in goverment that supports changing the law. Good example would likely be the industrialists.
Certain production methods give you more pops more likely to be an industrialist.
Early game clothes industry can get dye workshops production method for example.
Getting rid of peasant army and things that give landholders bonuses will give other groups more influence in comparison.
You can also bolster and suppress groups. Thinks its the government screen and you right click the group? Cant remember exactly where the button is right now.