Russia has already started that they would recognize and support a Republic of Texas. I'd venture to say that if Texas did, other states would follow and would form treaties.

I do not believe that the civil war settled the legal debate over succession. it is unclear, just because the union fought and got them to rejoin doesn't change the fact that succession isn't mentioned in the constitution.

the better solution IMO is a states convention amendment that spells put the process to remove. OR makes a state unable to leave from that point on.

I do not believe a union can allow voluntary admission, without saying they can't leave explicitly, and then not allow it.

Just like Brexit, there must be a way and process, legally.