Slavery, and follow on policies of indentured service, crop sharing, etc are the only reasons the pro-slavery states maintained economic relevance as long as they did.
There isn't a good mechanism to educate slaves; a key plank of the modern economy is letting people sort themselves into the role they can be most productive in.
You can't find an example in the last century of an economy of slaves outperforming an economy made up of the free. Economists would uniformly disagree with you; your ideas of what make most economic sense are unprofitable vs a no-slavery economic policy.
Essentially you've got a large debt. You're only allowed to work for the person who you owe that debt to. That person gets to choose your interest rates and your pay rate. For many it is made impossible to ever repay that debt, even if they pay it many times over.
Note they can't go somewhere else to get a higher paying job, because to do that they have to pay off their debt.