If all the Confederacy desired was the perpetuation of slavery, all they had to do was to remain in the union.
With the Dred Scott case intact, the Corwin Amendment sent out for ratification, and Lincoln's endorsement...which meant that there was no governing authority to threaten slavery, then remaining in the Union meant less risk.
The facts contradict your conclusion.
The south seceded to keep slavery in spite of Corwin and other offers to retain slavery but limit it to the states that already had it. The south could have stayed with the union by accepting Corwin or one of the other similar offers that were supported by Lincoln. They arrogantly decided that even though Lincoln supported Corwin and would not declare slavery illegal by presidential decree. The south acted like the Palestinians who have been repeadedly offered all of their demands yet that still isn’t enough.
They had simply to stay with the union with multiple assurances that they could keep their “peculiar” democrat institution. They chose secession and starting a war because the great satan Lincoln had been elected and they wanted slavery only on their terms, in all states.
I don’t see any facts that contradict the conclusion that for the south it was all about slavery.