I think the class character of a movement informs whether it should be supported, and in this case Catalan independence is mostly rich people trying to break away to pay fewer taxes and thus shouldn't be supported. Imagine if you allowed this type of secession on principal every single time it came up: large chunks of Europe would be sectioned off from the rest and turned into ghettos while the rich parts continue to siphon wealth from them and the rest of the world in an intensified version of the modern system of unequal exchange. Not exactly an improvement unless you're an accelerationist.

It's different from supporting a problematic nationalist movement against imperialism, ie Iran or Russia or Palestine. Even if the people of those countries have fully developed class consciousness, fat lot of good it would do them if they were fully under the thumb of a much more powerful country. The people of Spain are not under any such imperialist thumb, and thus only their working class movements should be supported.