> As the UK proved, a nation in the EU is free to leave the EU, but a state in the US is not. (Without consent of the US, of course.)

Which article or amendment of the U.S. Constitution forbids a state to leave without permission of the U.S.?

There is of course no clause of the constitution that forbids secession but SCOTUS currently interprets the constitution as creating an "indestructible union" if I recall the language correctly.

And good luck finding a good vehicle to overturn that precedent.

The TEU at least provides an explicit process for leaving