And 20% of them will turn out trans. What do we do with those people?
Again, that slippery word "trans". But here from context, I deduce you to be using "trans" to mean "their gender dysphoria persists beyond adolescence".
In which case we go on to treat it, as we always used to, primarily via therapy, like any dysmorphia about a healthy body. When there's nothing physically wrong, the cost-benefit analysis of any physical treatment strongly pushes against physical intervention.
Maybe, as an adult they could choose to undergo some sort of surgical or hormonal "transition". But the jury is out on whether the NHS should be providing that - the evidence base does not seem solid on whether it actually is of benefit as a medical treatment for gender dysphoria. We don't do the same for other dysmorphia conditions - people who don't want their arms or legs.
But as a cosmetic procedure, there is a limit on how much society can stop adults doing things to themselves. I can imagine tighter regulation on those carrying out the procedures, at least, and much tighter regulation on marketing that might be encountered by youth like "yeet the teets".
We just want this stuff to be treated like all other medical and/or cosmetic procedures, looking at costs and benefits, and restricting marketing.
Tell them the world would be better without them?
You can tell them the world is better off without them if that's the way you feel, but I doubt it would help. It's certainly not the way we feel - like everyone, they should be given the best possible care for any condition they may have.
The only people having fantasies about "trans genocide" are those claiming to be their "allies". I'd be beginning wonder if they had my best interests at heart if I had gender dysphoria. Why so many lies?