The hatred for people who still like what she created and still have fond memories is absolutely wrong, let's get that out the way first thing first. Just because anything turns sour, doesn't mean you're not allowed to have fond memories of it and when it wasn't and the fun times you had.
I think terms "Cancel Culture" need to be used carefully though. After all, JK is not cancelled. She just had a book come out. She still has a massive platform and lots of money and will likely continue to have for quite some time. She's not been cancelled, she's doing what she always does. (For those unaware, JK has a history in the UK of doing/saying something offensive/dumb and then when reported, sets her massive expensive legal team on people until they drop it and apologize or go broke to the point where a lot of new outlets wouldn't report on her).
But, as you say, she has the right to be offensive and say what she feels, and we have the right to disagree and not by her books and movies and no longer support her. And we have the right to tell her what we think. Unfortunately, that can be done in a nasty way, and then she takes the high ground because she didn't lose her temper, so is right. But JK seems to take offense to the very idea of being told she's offended people or is wrong (and let's be clear, she was factually wrong about several of her tweets).
JK is not like say Lovecraft. Lovecraft is dead, he's not getting any anything from us buying his books (unless his books were more real than we think, in which case we should definitely buy his books and prepare for the horrors to come) and our criticism and boycotting isn't going to teach him a lesson. But JK is still alive and still getting money and still has influence and a huge platform. We can do something to tell her we, as her customers, don't approve. As is our right.
Of course, she's so rich nothing we do as consumers will really make a difference, she never actually needs to publish or work again, but it's the principal that counts. There's no real clear answer to separating artist from art in some of these cases.
And also let's be clear, twitter is not a good place for these discussions, it's not designed for nuance. Even a polite answer in so few characters can come off as rude and snippy.