
While i do agree with you, you’re writing off a lot of other reasons they wanted to secede from the US. One big reason was the election of 1860 where Lincoln won without a single vote proved that the population density of the North could now dictate national leadership without any Southern input.
it was the Three-Fifths Compromise (3/5, not 2/3), but thats 0.6 compared to 0.667. and it did the exact opposite of what you're suggesting. Counting 60% of the enslaved population gave the South artificially inflated representation in both the house and the electoral college for decades. And without that extra political leverage, northern population density would have completely dominated national elections long before 1860.