...even without the issue of the African slavery ( which was of an industrial-scale and brutality that makes it quite unlike most other historical slavery )...
If I am understanding you correctly, that you think American slavery was worse than any other slavery and that that is what makes it "unlike most other historical slavery", you need to go and brush up on your history. American slaves were treated like royalty compared to the largest slave nations throughout history. Here's an example: "Spanish historian Fray Diego de Durán reported that 80,400 men, women and children were sacrificed for the inauguration of the Templo Mayor under a previous Aztec emperor" (source:
https://www.history.com/news/aztec-human-sacrifice-religion). They cut the still-beating hearts out of the chests of men, women, AND children. I'm not saying slavery here in the US was a cake-walk; on the contrary, actually. But don't try and spout off that it stands alone as the most brutal slave nation in history.
