You are quite correct that southerners seceded primarily because they were in the process of losing control of the federal government.
The ironic part, of course, is that their loss of control was precipitated primarily by their desperate attempts to tighten it. The Kansas-Nebraska Act and the Dred Scott decision were attempts to put slavery beyond political discussion. They led directly to the formation of a specifically anti-slavery party, which had not existed before. Up to this point, both parties had included both pro=slavery and anti-slavery groups. For instance, David Wilmost of the Wilmot proviso, was a Democrat. He eventually became a Republican.
In the latter case, southern justices suddenly discovered in the mid-1850s that there was a constitutional right to take slaves anywhere in the country, and reside indefinitely without it having any effect on the slave’s status. This was despite nobody having suspected any such right in the previous 75 years of the Constitution.
This was remarkably similar to pro-abortion justices suddenly discovering in the 1960s that the Constitution held a previously unsuspected right to abortion on demand.
In both cases the Court was attempting to put a political issue out of the realm of politics. Both backfired, making the issue mopre of a political hot button than it had been previously. With minor exceptions, Roe hasn’t led to shooting yet.
This is why I say the Dred Scott decision, in effect made slavery legal in every state, regardless that's state's laws on the subject.
It's a key point that nobody -- nobody -- understands.
In the late 1850s, the Slave Power was on the march, it was victorious, within a hair's breadth, within one more Supreme Court decision of having slavery declared constitutionally legal in every state.
Already in 1857 the Supreme Court said a slave-owner could take his slaves anywhere and maintain ownership.
So what was to prevent a slave-owner who brought slaves to, let's say, New York from selling his slaves to someone else while there?
And that is what had Northerners so outraged and enraged by the time of 1860s election.
What the Slave Power needed to do then was play it cool, give emotions time to settle down, and then work quietly to nail down their successes with further court decisions.
But unfortunately -- or fortunately depending on your point of view -- instead they took a path guaranteed to split the Union and start a Civil War.
The reason is the work of Southern Fire Eaters.