>>17898751 (OP)
I guess the main question I'd have is how the South would prevent slaves from fleeing northwards. The Fugitive Slave act meant slaves had to be returned, at least on paper. Now the free states in the North have no obligation to respect Southern property laws. Once the slaves reach free soil, they're free.
So you have a hostile power to the north, and millions of slaves know they simply need to get to the Union instead of Canada, which is far easier logistically for the underground railroad. It'd be interesting if there was an armed slave rebellion. Would the slave states be willing to deplete their own garrisons and send them over? Like would Virginia be willing to deploy soldiers to suppress a slave revolt in Georgia? We saw in the Civil War a lot of CSA soldiers and states were hesitant or outright resistant to sending their troops out of state. If Virginia legislators saw blacks slaughtering Georgians, they'd rather keep their armies at home to suppress any possible revolt there than send out their best weapon.
Agricultural economy means drought or a severe weather incident can fuck over their income. Might even lead to famine in the worst case, which would ferment possible armed slave revolts.