Religion is a belief system because it’s defined by belief; trans is a demographic category because it’s defined by lived status.
No, trans is defined by a declaration. Is trans whoever declares being trans, not whoever has a "lived status" of trans. Similarly, religion is about a lot more than just belief: it's also about living in a specific way. I've been both religious and trans, and I know that if anything, being religious had a far greater affect on my life than being trans.
They describe trans people as a population group whose experiences and outcomes can be studied — the same way they describe groups based on sexuality, disability, or ethnicity
Or religion. You are aware that religion is a population group trait that is also studied, right?
Sexual orientation is a perfect example: you can’t measure it in a lab or a skeleton, it’s self‑described,
Who people have sex with is an observable trait. When someone says, "I only have sex with men," they are reporting an observable behaviour, not some unobservable belief about themselves.
You still haven’t explained why every major medical and psychological body rejects your view — you’ve avoided that entirely.
Not at all. I've expressly asked you what view those bodies support that GC people don't. Since you didn't answer me, I can't tell you why they supposedly reject my view.