I agree that you shouldn't do anything to actively contribute to Rowling and her platform. I'd mostly disagree that creating fan art or fic supports her. In fact, I think she'd be pissed as hell to see all the Trans HP [insert character here] art and fic that has flooded the internet since she made her TERF stance clear, to say nothing of the "hey I hate [insert problematic aspect of these books] so I'm going to change it" flavor of fic that has existed since the book series wasn't even complete (exhibit A, the HP fic I wrote a few years back).
Fan works are not created in a vacuum without context. If people are saying they do not support Rowling's ideology, but they get joy out of taking her characters and narratives and world-building and changing them to explicitly subvert or correct racist/homophobic/antisemitic tropes, I find it difficult to rationalize that as support for Rowling. Someone new to the fandom isn't going to find a fic/piece of art with an author's note that says "hey I wrote/drew trans Harry because I hate JK Rowling and want a way to reclaim HP so that I can still enjoy it" and think "oh, neat, I should go buy the original books from Amazon and support Jk Rowling."
Henry Jenkins said: “Fan fiction is a way of the culture repairing the damage done in a system where contemporary myths are owned by corporations instead of by the folk.”
I agree, but I also think this can be extended to repairing the damage done by individual authors and their biases.
Basically, I think fandom furthers the idea of the "death of the author." In this case, fandom isn't just saying the author is dead, they're saying they killed her and now they're going through her belongings and throwing out the stuff they don't like while keeping/repairing/adding on to the things they do.
So, if you get joy out of making HP fan art or fic and you're actively trying to discourage people from supporting the original creator, and encouraging them to be critical of the original content's failings even while enjoying the fandom, I find that far more admirable than problematic.
As is usually the case, when people online are telling you you're a terrible person for doing a thing you enjoy that harms no one, they're likely the ones that need to go touch grass and get a little real-life perspective.