Considering what a stupid fucking fad it is nowadays I'd react with extreme skepticism. LGB is no big deal, they'll probably give it up in a couple of years and if they don't, who cares, the only family members old enough to give a fuck are all dead now anyway. T is the real problem, if it's just the "nonbinary" nonsense I can pretty much ignore it. I wouldn't bother to fight letting them dress however they please (laughing at the way teenagers dress due to stupid fads is a time-honored tradition) but I'm not indulging any pronoun nonsense at that age. It would probably be fucked up if I denied them therapy if they wanted it but I'm so afraid of the mental health community enabling this shit I'd be hesitant and very careful, no hormones and certainly no butchery under my roof, that's for fucking sure. I am not taking them to a "gender clinic" but I guess I'd get them in to see a psychologist who specializes in anxiety/depression/identity issues for teenagers and just hope they aren't troon enablers.
In the extremely unlikely case they stick with the trans shit for life I guess I'd learn to live with it and start making an attempt at correct pronouns but I'm not enabling it (helping with changing names/gender on ID, giving money for procedures, etc). Deviancy over the internet will not be tolerated and if I catch them doing anything fucked up all phones, computers, etc will be taken away and cut off until they're able and willing to pay for that shit themselves. Many talks will be had about the unfortunate tendency for people nowadays to center their entire identity on their orientation/"gender"/mental illness and how bogus that is, I know that kind of thing is normal to an extent for teenagers but if I keep addressing it regularly they'll hopefully grow out of that shit by college.
The other piece is emphasizing the importance of being very careful about internet anonymity and how every cringey thing you do is out there for good, you have to start young with that. I think in some cases people stick with the LGBT phase (or "illness" phase or whatever) longer than they would have otherwise because all the shit they've posted about it online makes it more difficult to "reinvent [their] identity" as it were.