I believe very strongly in bodily autonomy for adults.
The price of living in a free society is that others can use that freedom to do thing you find monstrous. I think it's a price worth paying.
In theory I agree with you, but the older I get the more I have to accept that theory and the real world don't always match up.
People do make bad choices. Sometimes they very deeply believe things that are not true. About themselves, about others, about the way the world works, about what is and is not possible. Sometimes they misattribute their own motivations and try to solve their pain with things that smother it instead of things that heal it. They do things to be accepted by others, or because they feel they backed themselves into a corner or into a public image and can't back down without (what they feel to be) an unbearable loss of face or status, or because they do not want to let down others who believe in them. And that is just what people do to themselves without even starting on the ways others can, for their own fucked up reasons either deliberately or accidentally influence people to make bad choices. Or Inauthentic choices, if "bad" is too loaded.
So because people do make choices in good faith that turn out to be mistakes, the more damaging the choice, and the more irreversible, the higher the bar should be.
But even if we do agree that eventually, once all reasonable checkpoints are put in place, medical adjustment of the body to visually approximate the opposite sex is still a decision people need to be allowed to make, there is still absolutely no justification to then treat those people as the opposite sex, because they are not.
Their choice to appear to others as the opposite sex, allow others to believe they are the opposite sex, even lie to others about their sex, is theirs. That does not, and IMO must not, place any moral obligation on society to play along by giving them any legal or social right to access provisions or rights of the opposite sex.
Firstly, because that makes a mockery of the reason those provisions and rights exist in the first place, which is because the people who actually are that sex faced challenges, risks or disadvantages that single sex provisions mitigated. Single sex protections and provisions were not put in place out of some arbitrary whim or fashionable affectation for sex specificity, they were put in place as a response to the specific disadvantage or damage of women by men. They aren't there to be some sort of magic sorting door to prove ones womanly credentials!
Secondly, because there should be no "reward" for extreme body modification that might influence a person's decision. If a person feels very strongly they want to exercise their bodily autonomy to make those changes, and has cleared whatever protections we think are proportionate, that is between them and their private life. It should not be turned into a socially or legally recognized actual change of sex.
The reason GRCs and their equivalents in other countries do not require any medical interventions is because it was considered to be unethical to require a person to undergo surgery. I entirely agree with that. But the logical conclusion should have been to recognise that the entire concept of a "real" (ie legally meaningful) sex change was flawed and should be dropped.
That TRAs managed to convince lawmakers that the solution is to turn legal sex change from something that recognised a person had done as much as could be done to change their physical sex (a certification that recognised the material state of the person) into something that accepts into law that whether a person is a man or a woman is an entirely mental difference and one that comes down to personal choice is IMO unconscionable.