AMERICA is to be renamed

I don't see how you can completely separate racism from the 30's onward from before that. Again, the people forced to give up slavery resented the forces that made them do so, as well as the former slaves themselves. You acknowledge this now, but are not looking at the full ramifications of what this would mean (i.e. how it would play out over time).
They immediately tried to replace slavery with things like debtor's prisons (where they had many right back on the plantation) and other forms of discrimination, and even violence. Blacks had managed to make gains as great as becoming governors (fulfilling General Benning's dread!), but when the North pulled out, and their influence was removed, this was quickly halted! The KKK arose, terrorizing not only blacks, but also those sympathetic to them. They created the stereotypes of the blacks being dangerous violent criminals and racists to justify all this (from Benning, again: "The consequence will be that our men will be all exterminated or expelled to wander as vagabonds over a hostile Earth, and as for our women, their fate will be too horrible to contemplate even in fancy"), and produced propaganda like Birth of a Nation promoting this and the KKK.
That now is into the 20th Century. This just continued through the 30's, and resentment grew then and beyond, as they were forced to make more and more concessions on race.

You said you always found a bit odd that some spoke out against the "evils of slavery". Well, some people knew it was wrong, but societal pressure still led them to go along with it. If people in high places like Benning believed as they did, then when slavery ended, they did not just drop those beliefs and fears (and the whole "black" issue), only to be suddenly picked up again in the 30's.

And since people say "God uses all things for His glory and people need to realize the larger picture." regarding slavery, then why don't they see it that when when it comes to changes they don't like. So God "used" slavery (to save them from their tribalism), but taking down monuments (that offend people) is the work of the Devil! Why?
(Though I always agreed with the last point, that racism was man elevating himself over another not to downgrade the other but to elevate himself. Still, the "net effect" is the same).

 

How did they “lose relation” to one another? The people clearly stated black inferiority in the days of slavery and the Confederacy, and they did not stop believing that, and then start believing in it again in the 1930's. People passed down belefs, feelings and the overall resentment from gerneration to generation. Remember the slogan, “The South shall rise again!” They did not drop that sentiment in the 19th century.

From something you said earlier, this splitting 20th century racism off from slavery is obviously another attempt to pin all racial problems afterward on the Democratic Party. Yet it's the people more opposing the Democrats who more often spew classic racist stereotypes (of those bygone centuries), such as the blacks only wanting “free stuff“ (i.e. “lazy”), and all the crime (“dangerous”), etc. and now framing it as concern for the people, because it's all the Democrats' fault for taking advantage of their desire for “handouts” (like they're easily duped as well). This is what people are finally recognizing now, and why they reject the other party.

Trying to isolate it to just a party is playing politics, not trying to offer a solution. Any party can be racist, and many blacks certainly question the past history of Biden. But all either is doing is pointing the finger back and forth.