If you read the text of the Declaration of Independence, it makes it clear that the listing of grievances is a courtesy, not a requirement.
...a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
The Declaration of Independence states that people have a God given right to leave for whatever reasons they so choose.
That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, — That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.
The Declaration of Independence (D of I) is basically a treatise to the world about what justified the colonists’ secession. It is probably the most elegant and well reasoned justification for secession maybe in the history of the world. It offers justifications of that which “Prudence, indeed, will dictate...” offered for consideration to a “candid world.”
The D of I gives instruction and guidance, not legal or constitutional dictates, for valid secession. However, IMO, the D of I has persuasive authority because of its integrity and influence in American law and culture.
The D of I shows the steps to valid secession and that secession:
1) should not be “for light or transient causes”
2) requires a certain “patient sufferance” while “evils are sufferable”
3) involves notifying and submitting the facts of abuse “to a candid world” (27 specific abuses are listed in the D of I) and finally
4) “when a long train of abuses and usurpations, pursuing invariably the same Object, evinces a design to reduce them under absolute Despotism, it is their right, it is their duty to throw off such Government.”
The colonists suffered many decades of harm from George III and continually notified him of his wrongs and pleaded for redress. In contrast, the South had not yet suffered any unconstitutional acts from the feds regarding slavery. The South should have first notified the feds of what acts were unconstitutional and why they were unconstitutional. Instead they ceded in anticipation of federal acts. There was really no “patient sufferance” and no attempt to notify the feds with reasonable constitutional arguments.
So the South’s cessation was invalid IMO. I believe the North had a constitutional right to fight them and get them back into the Union.