I agree completely with your analysis.
Just speculating, I’d imagine that “leaving the EU” they would find to be in gross violation.
Well, yes. And that’s in spite of their lengthy “withdrawal from the EU” process, which gives all the negotiating advantage to the central government.
For the record, the USSR also had a brief clause in its constitution that allowed any member of the USSR to “freely secede”. In practice, that was certainly a different story; just ask Hungary.