Did you like the subplot about how slaves who are freed against their will turn to alcoholism?

Yeah, I thought it was really interesting how there were two characters who gained freedom and handled it in completely opposite ways. I thought it was a great way to highlight that simply ending an injustice often isn't enough. It takes effort beyond that to truly reach justice/equity.

Or how when they celebrated Christmas at Grimmauld place, they put little santa hats and beards on the severed slave heads?

The severed heads themselves were clearly established as one of many things that made everyone being forced to live there uncomfortable. So, yes, I liked the touch of the characters decorating them, and the rest of the house, to try and make it less of a reminder of the shitstain of a family that it used to belong to. The characters make quite a few such attempts, throughout the book, often unsuccessfully.

Did you like the HIV allegory character who deliberately tries to infect young boys with his disease?

Yeah, it's a pretty terrifying concept, and a great lesson about how being a victim doesn't make someone good. Anyone can be evil. In fact, victimization often becomes the SEED of future evil.

What about the constant descriptions of “mannish hands” and general authorial misogyny against women who the reader isn’t supposed to like?

I don't see how one instance of the phrase "mannish hands" across seven books equates to "constant descriptions". I can't say that I liked it or disliked it, because I don't ever remember reading it. It wasn't a significant enough detail to remember, just descriptive flavor of what the author was picturing. In retrospect today, yeah, that seems like anti-trans bias subconsciously leaking out, to have a "bad" woman character have masculine qualities. But it definitely doesn't read that way, on its own.

Did you like how Harry was supposed to be the saviour of magical england from a fascist movement, and yet he’s a moderate liberal who never makes an effort to fundamentally change any of the systems of the world, and who wants Hermione to stop campaigning against slavery because it’s annoying?

Given that the books actually give zero picture of how much magical society has changed, after Voldemort's death, I don't see how I can answer that. The only thing we know for sure about the world is that Hogwarts and Platform 9 3/4 still exist. I could give a fuck about what Rowling's expanded on in interviews and musings on Twitter.

I don't recall Harry ever once being against SPEW, that was pretty much all Ron, who does eventually change his mind. What Harry DOES have is the fantastic story arc with Kreacher, where he explicitly recognizes how wrong he was to not see the barbarity of the system sooner.