As a kid I read the first few Harry Potter books but gave up the series somewhere in the middle of the Goblet of Fire, when it started getting too elaborate and teenage angsty and Rowling started to write this epic hero's journey about something or other. Didn't think she's good enough of a writer to carry that to completion, not that I ever went back and checked...
As for the transphobic stuff, it's obviously very disappointing. I have some empathy for her in that given her childhood/upbringing, but that doesn't excuse wilfully spreading misinformation that damages the lives of thousands of people because it reconfirms your priors. Sad to think about.
Seeing that transgender people who get sex reassignment are still massively more likely to commit suicide than non-transgender people, it does raise the question, whether sex reassignment is the correct treatment. In this it's a major problem that any such discussion end up being called transphobic.
On the other hand Rowling didn't help her argument about her not being a TERF by writing her latest book.
For Rowling to be a TERF she'd have to be a radical feminist in the first place which she is not, she's a mainstream liberal feminist. And the controversy over her latest book is blown way out of proportion, the mention of a cis male crossdressing serial killer happens once as a reference to a cold case. It's not a main character or big role in the plot. Nor is it even trans-related as the killer is still a cis male.
The thing about whether she's a TERF is semantics. Who cares whether or not she's a radical feminist? The main problem with TERFs is not that they're radical feminists, and she's certainly not a "mainstream liberal feminist" given her views on trans people.
As for the book, "scary man dresses up as a woman" is a common transphobic talking point. Its inclusion in the book is not terrible on its own, but this book does not exist in a vacuum.