I'm not an advocate for a trans sports category
Yeah. Me neither. If someone believes “transitioning” is necessary to their mental health, then they need to accept that some kinds of competitive sports will not be open to them.
We all make decisions which have life-changing consequences; choosing not to go to university precludes being able to undertake some careers; having children precludes living an unfettered life - and “transitioning” should preclude competing in the teams which match gender-belief.