So I agree with the first half pretty well, you make some good points. But:

there’s nothing inherently gatekeeping about it;

Yes there is. It’s defining folk who medically transition as being a different class of trans folk. We’re not a different class. We all of us have unique needs, and the language should focus on those individual needs, whether they’re medical, social or other.

In general, just because everyone has unique needs/qualities/etc., that doesn't mean that it's not useful to have categories anyways. Although in this case perhaps you're right, the situations are often complicated enough that it would be too reductive. In extending my wider pro-categorization stance to this issue in particular I may have ignored the naturally complex nature of it.

I get lumped in with people who genuinely believe statements like this.

I'm not sure exactly what you're referring to, and you reacted differently enough compared to the rest of what I said that I think you may have misinterpreted my stance here?

It denies people the right to their own identity. That is inherently bad.

And that's why I started this off by saying that it wouldn't be productive to argue for this. Even if I were correct in theory*, nobody who this matters for would ever accept my definition, or any definition, other than the one that they believe to be true. You cannot force someone to accept a label that they don't want, even if there would be benefits to using it. Although given what you said I'm not sure now that there would be benefits anyways.

*as far as that could apply to language, anyways