The times self determination was a win: Finland from the Russian Soviet Federative Socialist Republic. That went fairly well, at least as things can get. Even better was Norway in 1905. Times territorial integrity might have been a better idea: Czechoslovakia 1938. Oof, that was pretty bad in the end.
Sometimes countries actually have it in their laws that a member may leave if they wish like the Soviet constitution ironically enough and the Yugoslav one, and for some reason the eleven villages of Liechtenstein can leave, as if Liechtenstein wasn’t small enough after Bismarck had his way with Austria in 1866.
Europe in the last 230 years since the 1789 revolution in France can to some degree be summarized as the push and pull between these two sides. What do you think is the right balance between the two options?