To me, the Hermione thing had always felt like JK was trying to make her out to be a "blue hair/pronouns" feminist who shouldn't be taken seriously and in the process she accidentally walked face first into making the same argument that actual slave owners made to justify themselves. I don't think she intentionally meant to justify slavery, but she ended up there trying to criticize Hermione.

This is why I recommend Shaun's video to people, as it tries to take an impartial look at the books. He points out how it feels like JK's point of view shifts as the books go on, and she goes from criticizing the system to defending it as the money started rolling in and she began to benefit from that same system. But there are some constants with her open bigotry now even as far back as the first book, some of which I've already mentioned, like the stereotypical characters (which could easily come from growing up in a sheltered environment, but she claims to deeply research a culture before creating a character) and applying masculine traits as a negative to female characters. Whether or not she supports slavery we can only guess at as she's never made a statement on the subject, and I can't imagine that she does, but her bigotry can be seen to not be a recent development, just a more deeply entrenched or worsening belief.