Confederates as a modern-day North Korea?

Well, I tend to be skeptical about the long-term liveability of a pariah state at North Korea levels of isolation. As a matter of fact, NK itself survived so far just because China massively subsidized its economy for its own strategic interests. Short of drastically restructuring its economy, which was oriented on commodity exports, I am even more skeptical the CSA could survive for long in a situation of extreme international isolation. On the other hand, it is far from impossible to think of an ATL situation where the CSA manages to tone down international hostility at least to apartheid South Africa levels (which demonstrably resisted domestic and external pressures for reform for decades without having a great-power sugar daddy) and/or reaps some meaningful international support.

As a matter of fact, I have been recently considering this kind of scenario. A CSA equivalent arises in Southern North America w/o having been part of the USA and with American benign neglect (at least at the beginning); Brazil picks a far right evolutionary path and clings to slavery as much as possible like the CSA, becoming its South American clone; South Africa gets a major influx of reactionary-minded White immigrants early in its trajectory (say American Loyalists first, refugees from successful radical-democratic 1848 Revolutions in Europe later) and goes Draka on Southern African Blacks. The three states establish an informal trans-Atlantic "Axis of Slavery". If and when chattel slavery turns out truly unsustainable they just switch to hardcore apartheid instead. A great power or three turn hardcore right-wing authoritarian and have few scruples supporting a few middle-power allies/clients with slavery or an apartheid regime, esp. if they just victimize the ethnic group consistently at the bottom of the global totem pole these last few centuries. It might be a successful Axis, a fascist Russia, and/or a fascist China. No strict equivalent of the Holocaust occurs or gets widely publicized to shock public opinion against racism at large by using the typical methods of colonial genocide against Europeans, so prejudice against Blacks remains fashionable or at least tolerable much longer. Of course, in my own planned scenario a few other powers turn more liberal than OTL to try and balance the scales of dystopia, the general TL concept is a global liberal vs. fascist/slaver Cold War that does not concern victorious Nazi tropes, but you get the general idea.

Apartheid South Africa was already dangerous with it nukes,cannot imagine how a CSA with nukes will be.

The Confederacy would likely be a major power, so I'm doubtful the situation would ever reach the extent of North Korea.