I won't say you're wrong - it was largely about independence and state's rights. But the focal points that they chose as their reasons for secession were what ultimately doomed their efforts, and slavery was a huge one. The tariffs probably ran a close second, being essentially intranational economic warfare. But what else? What reasons did they truly have to secede? What rights of theirs were truly being violated?
History is not kind to rebels without causes. Heck, I daresay we've more reason *today* to secede than the South had in the 1860's.
But neither of our straits compare to what our founders endured before finally writing the Declaration.