Since whether secession is or is not a legal right of the states has never been a topic I’ve weighed in on, it’s irrelevant to me. My only dog in the fight are these points:
The reason for secession was the South’s desire to preserve slavery.
The trigger for secession was the election of Abraham Lincoln, which fed fear of the abolitionist foundation of the Republican Party.
The American Civil War began when units of the South Carolina Militia fired upon a Union fort in Charleston harbor.
If you are discussing the Civil War, you are weighing in on this topic. The entire justification for invading the other states was based on the assertion that states did not have a right to leave the Union.
If you won't defend that argument, you aren't debating honestly.