You don’t think the founding fathers that were alive when states started passing laws to end slavery in their borders wouldn’t have said something about it being unconstitutional?
I think what George Washington did in fact do speaks volumes regarding his opinion of the matter. He clearly flouted the law as much as suited him.
But why does this discussion always come back to slavery, when the right to have independence does not require some justification approved by others?
Focus on slavery implies that if they had better reasons, then their efforts to secede would be accepted.
Is that what you are arguing? That their effort to break away would have been okay if they had better reasons?