The 5th Amendment in the United States is a constitutional amendment that says you don't have to testify against yourself in a trial--you do have to tell the truth, but you can just remain silent rather than being forced to implicate yourself in wrongdoing. I was using it allegorically.

Anyway, with your additional clarifications, yeah, maybe there's something to this as a definition. It would mean something if taken seriously (e.g. we pay attention to trying to define and detect and study subconscious sex). Is it the right distinction? I can't tell, but it could well be worth exploring further to figure out.

I don't think that you can go entirely with "trans people define who trans people are" without basically falling into the same circularity as "a woman is anyone who says she's a woman" situation, however. Members of the (hypothetical) We Agree You're A Member Society are people who agree that each other are members...but that doesn't explain to anyone else why WAYAMS membership is relevant to anything. Same deal with neurodivergence inasmuch as neurodivergent people ask for anything (even just understanding) from anyone else. It's reasonable to have someone with personal experience act to more sensitively assess who is whom, but if it's not explicable to everyone, it's a big ask that others should respect requests for non-trivially differential treatment. ("Members of WAYAMS should be greeted with 'yo', not 'hi'"--sure, as long as it's not a big deal if we sometimes forget or don't really feel like it. "Members of WAYAMS should pay 50% less taxes"--hang on there!) But unlike the woman-identifies-as-a-woman definition, yours could at least potentially lend itself to explanation to people who aren't in the group. (And aren't on the reactionary right--they're not listening; I don't think they've even gotten over letting women wear pants yet.)

That would be a significant improvement over the current state of affairs, which seems to try to explain things to people like so: "Gender dysphoria is a distressing and potentially life-threatening condition that is recognized by the medical establishment and can be treated by gender-affirming care, so therefore a woman is anyone who says they are a woman." For people who have only the attention-span for a soundbite, that's about what they get, and it's not the most convincing message.