“A reasonable perspective. Except that the only states’ right southern leaders really were concerned about, or that they felt was threatened was the right to own slaves.”
That’s overly simplistic. The North had an advantage in both population as a whole (more urban) and as apportioned (3/5ths rule). That led to the North having a veto over the South in Washington. Add to that the contention over slavery and every policy disagreement had ominous overtones.
For example, applied today, the biggest issue for advocating States’ Rights is limiting government. While you can say that overarches much of our current politics, the recent Supreme Cabal decision is evidence that there are more issues at play.
You can equally accurately say the South got a boost in representation by the 3/5 rule.
The South claimed slaves were property, and was increasingly moving towards the notion that they weren’t really men at all. (This is, after all, the only way to square the sentiments of the Declaration of Independence with slavery.)
So why should southern property be counted when determining representation while northern property was not?
One could say that it was the South which benefited from apportionment since 60% of their slave population, which had no need for representation in Washington since they were property and not citizens, was counted when deciding congressional representation.