For those unsure about how fandom contributes to JK Rowling's bullshit. I've been working in digital marketing for over 15 years and this is how it works:
Harry Potter fanfic indirectly supports her by keeping her IP alive and strengthens her branding.
Think of it like bookstagram or booktok, if people keep talking about a book, more people will read that book and fanfiction works the same way. I've had tons of people tell me they've started watching shows because of fanfics I've written. It's the same for other fanfic authors too including Harry Potter fans, and that is a huge fandom, one that keeps churning out work that keeps her relevent and that relevence is what leads to people to seek out the source content eg: the books, the movies, watching the show (she currently stands to net around 20 million a year for this).
You're basically a cog in the machine that keeps her making money.
People leaning on the nostaligia aspect are using it as an excuse not confront the bigger issues at play or because it doesn’t affect them directly. (eg not UK based so aren't feeling/seeing the real effects, don’t have friends/family members who are trans so they support trans in theory)
I know it's complicated for a lot of people because ‘I grew up with Harry Potter’ or ‘it helped me through a bad time’ – it did for me too. It was the thing I was reading to escape a very traumatic and messy homelife.
For me I find it hard to reconcile my enjoyment of Harry Potter with the knowledge that in some capacity (reading it, writing it, buying shit) I would be supporting a person who is actively trying to hurt people that I care about. Why would I do that? Why would I want to be around other people who choose to do that?
You have to question, if you are willing to do that, are you really the ally you claim to be?