"Pretext" is a matter of how you choose to see it. If you disagree with other people's reasons, you might call them "pretexts", but if you agree with them, you would recognize them as "causes."
I do not doubt that the South was fed up with being dominated by the North, and they were at that time wishing they hadn't signed on to the project.
But I think if you want a proper understanding of how they felt, (or as close as a modern man can come) you should read what John Calhoun had to say on the matter.
I find it interesting that you accept Jackson's claim that "the next pretext will be the negro, or slavery question."
You seem solidly determined to regard the slavery question as the sole force dominating all decisions by the South. This is hardly how you would classify something as a "pretext."
Myself and FLT-Bird have argued that slavery was a pretext. What they wanted was independence, and they thought the Union violations of the Constitution regarding slavery was a good legal argument for them to get independence.
I think Jackson correctly saw it for what it was, and he recognized they just wanted out.
Many wanted out. I've always believed it was primarily 3 reasons.
1) they correctly felt they were being economically exploited for others' benefit and would be much better off if independent.
2) they were tired of being pushed around by the North and had always been opposed to the federal government usurping ever more power for itself (since they knew they were always going to be the minority, this makes perfect sense)
3) Slavery. When I say slavery I don't mean they were worried about abolition being right around the corner. I mean they were tired of being treated as though they were immoral for having the very property those self righteous New England A-holes had sold them in the first place AND which they were more than happy to profit from. They were also tired of the whole agitation over the issue whether Northern states clearly violating the Fugitive Slave Clause of the Constitution, the violence in the territories over the issue, etc. Finally, when they did things like trying to start violent slave insurrections and arming and equipping terrorists like John Brown then refusing to prosecute or extradite those who sponsored the terrorist attack on them, that really tipped things over. They felt that these were not longer mere political opponents. These people really wanted them dead. They showed themselves to be actual enemies........note how this is not all that different about how many of us feel about the Northeastern Elites/Establishment from the self righteousness to the hypocrisy to the fact that they are real enemies rather than people with whom we merely disagree. Who the hell wants to stick around with that?
I recommend that you read "The South as a Conscious Minority, 1789-1861: A Study in Political Thought" by Jesse T. Carpenter. He traces the development of the compact theory of the Constitution and of the South as beleaguered and put upon as providing the legal and moral justification for secession.
Once such thinking became current in the South, secession became a clear possibility in the region's political thought. An unpopular war, a disputed presidential election, tariffs, slavery, or some other cause could then provide the pretext, to use Jackson's term.