Tl;Dr: From personal experiences, hold the line. Don't affirm.
Dsis and DBIL kept DN from a non-binary identity because they refused the new name, refused therapy and told her as long as she was living w/ them there'd be no new religious practices in their home. They were loving, but very, very firm. DN was a hot mess for a long time, including hiding under the table at dinnertime and barely talking. At 17/18 she started chilling out, they fixed some health issues (anemia and other deficiencies, and surgery for a chronic ear problem) and now DN is much, much better, no longer thinks she's a non-binary/trans or other gender thing, and told her parents she thinks she's a lesbian (such a relief and not surprising).
A friend was told to affirm DS at age 18 which she did, and now at 25 he dominates her, unable to keep a job, lives entirely off their income and turned from a sweet healthy boy into an angry, rude, obese man who thinks the world is out to get him. I recently told this dear friend how my Dsis and BIL helped DN and she gasped in shock that DN hasn't killed herself. I despise the professionals who lied to these parents.
About 5-6 years ago, DD (young teen at the time) tried to educate her old DM and DF about the trans issue. We died laughing because it was so absurd. It was a sore spot for about 3 years, including one time when DD cried because she was so worried I'd lose my job and we'd be homeless for my terfy views (she wasn't half wrong). But we didn't budge. Around 16, she rolled her eyes at a friend's 'elf/elfin/fae' pronouns. Somewhere around 18, DD acknowledged a soft light of reason in the distance. One day, she told us we were right, trans is nonsense, gender and non-binary are ridiculous luxury beliefs.
Later, she said she's so glad we laughed at it because she was used to us taking her seriously and having civil, respectful debates when disagreeing, so our reaction at this issue was so out of the norm, she couldn't fathom why...until she grew up. She'd taken a survey that said she was trans but was scared to tell us cause she knew we'd laugh at her. Today, she's a adult, and active feminist.
DD2, when discussed in school if parents would be accepting of a gender identity told a few friends with pride, "my parents would laugh at me. they don't take this seriously." And we live in a very progressive US town and I have progressive cred.
However, let me be clear: DDs don't have SEN or any mental health issues where a 'laugh my ass off' approach may not work. e.g. DN responded to firm, loving, undebatable boundaries. Regardless, hold the line. However that looks inside your family, hold it firm. It's the most loving thing you can do when the world outside is mad.