So, to go back to reframing:
I often think of the conversations I had with my lawyer friend who is a fanatical TRA, before we agreed not to fight about it. And this first emerged years ago when I was still basically in a "be kind" position, and self-ID was just emerging as the shiny new demand.
My argument was - maybe I reasoned myself into terfdom? - that if you try to apply full self-ID to equality law then you are opening a whole can of worms. Because protected characteristics depend on you being able to define the PC.
It runs right through equality legislation. Many PCs are just objective. You are the age you are. You are either pregnant or not. You can't be treated inequally because of this objective thing.
Sometimes it's contextual, as with chronic health conditions that might be relevant in cases of disability discrimination.
Sometimes it's based on perception. I'm not Jewish, but sometimes people think I am, and I'd still be protected from discrimination on the basis that someone thought I have a PC that I don't objectively have. The same goes for straight people mistaken for gay, Sikhs mistaken for Muslims, etc. Theoretically a transwoman would have the PC of female sex if discriminated against because people thought they were female - though I'm sceptical how often that happens.
But the thing is that there has to be an objective core to the PC. This also goes for protecting trans people from discrimination - you need to have a more or less stable idea of who is trans and who is not. I still think the GRA was bad law, but it was still workable as long as people understood there was a specific group to which it applied, and there was some gatekeeping.
Pure self-ID, based on ideas from queer theory, can't translate into anti-discrimination law because it means we have to accept that people are exactly what they say they are at any particular moment. Especially because nobody can really define what gender identity is. So applying the logic of self-ID means that not only is there no stable definition of woman, there's no stable definition of trans.
So for me the legal question comes down to - if you want to protect groups including trans people against unjust discrimination, you need stable definitions. It's not possible to have equality law based on the premise of "all these protected characteristics are really nebulous and nobody knows what they mean".