Secession is the orderly, negotiated dissolution of a compact. Rebellion is the organized opposition to authority or a conflict in which one faction tries to wrest control from another. What the Colonialists did in 1776 was unabashedly a rebellion. They openly challenged the authority of the king and crown. There was no pretense at “secession” and they knew that their very lives would be determined by the outcome.
The slavers rebelled in 1860 too, only they didn’t have the brass to be honest about their insurrection. Secession - as pretended by the slavers was illegal and is so today.
I will, once again, ask everybody on this thread to read the cornerstone speech, it mentions all sorts of well thought out principles of limited government and orderly secession, and debases it’s core with a defense of human slavery as a natural order created by god. It’s probably the best single document of the era that codifies all the political elements of the south - and is also the most honest.