I'm not surprised to hear your experience in the writing community. And yes, JKR is wonderful for this, not only as a woman, but also as a writer, the profession that should be most invested in freedom of expression.
There's a trend now for some publishers to have pre-publication novels checked by "sensitivity readers". These are readers who check to see there is no inadvertent racism, sexism, ableism, no cultural appropriation, etc, that readers might object to. Up to now I've not heard of sensitivity readers specifically for "transphobic" language, but if it ever become a thing they'd have a ton of work. A whole new career opens, since just about all novels written are basically transphobic, ie, they assume gender, they don't refer to their characters as cis-men and women, they assume that all characters are the sex they present as, in other words, they ignore gender identity. Imagine if every character needed an indication as to not only their sex but their gender identity! The ladydick could very well become a perfectly ordinary appendage to your basic heroine, along with a beard and adam's apple.
I have a whole lot of online UK-based writer friend and not one of them has raised the topic. I don't know who is GC and who not -- they never say, and all of the groups I'm in forbid political posts. But none of them have reacted to JKs tweet, anywhere. It's weird, the silence, since it concerns a writer, which we all are!
I don't know if I've mentioned this before but an older, artist friend of mine, who has ties to a retreat centre, said that recently, a theatre group from the USA came for a retreat there, and they were all trans and insisted on pronouns etc.
She, and the Irish staff (it's Ireland based) were forced into complying, which for many of the staff (just ordinary folk) must have been extremely confusing. It was her first encounter with the culture and it totally um -- let's say, it took her to the summit. Once we got talking she couldn't stop! :)
People in the arts are often extremely narcissistic and think they are on the cutting edge of cultural progress, so I suppose it's to be expected that the ideology can find a good foothold there. In my experience, though, we older ladies (like my artist friend) have our feet well planted on the ground. It comes with the territory -- many of us are grandmas and we tend to have a nose for bt.