I really appreciate this like of argument, and I don’t exactly diaagree, but at the same time I think you are falling into the same trap as the “simple biology” anti-trans crowd do, and looking for any one thing that defines sex. Sex is a complex collection of features which tend to co-occur. Your primary and secondary sex characteristics are all a part of what defines your sex. I’m not an expert here - the things I’ve listed are fairly basic in terms of what can/can’t be changed.

But there are many aspects that won’t change with transition - there is no treatment that will magically make me a cis woman, I will always be trans. I will always need to make new doctors aware that I am - because there are factors affecting some medications that mean I should be prescribed as a male for example.

I think the important thing for us trans people is not to focus too much on the biology. The important part imo is that it shouldn’t matter what your biology is - your gender identity is what makes you a man/woman. I’m wary of brain structure/chemistry “justifications” of trans identities for similar reasons. I’m sure there is truth there, don’t get me wrong! But I worry about over-medicalising trans identities in general, or even a diagnostic criteria which not all trans people meet…