To some extent, this is a fight over who is most oppressed. The fundamental issue is that if you acknowledge that transwomen are women, then they are oppressed for being women, as well as oppressed for being trans.
If you don't, then it's a bunch of men pretending to be women shoe-horning themselves into women's spaces. If you follow this line of reasoning, then it's these women who are being oppressed.
I can't think of any historical analogy at all.
The idea that a redistribution of rights is OK because it only affects a small number of people is very dangerous, and the GRA was vague on gender and sex - a wedge. So I agree with you when you say:
I’m still very much of the belief that gender identity and biological sex are separate things and I think if something could be put in law that makes that distinction clear, surely everything would be fine.
I'm not sure "fine" is quite the right word, but there needs to be a clear delineation in law between the two, with sex-based rights ring fenced. Most of people here want to fight the losing battle that "trans women are men," whereas fighting for sex-based rights does not require this.
The most dangerous (and wrong) TRA arguments, used as a wedge, are that sex is a spectrum, sex is as much a social construct as gender, and from that it follows that a transwoman should have the exactly the same access to protected areas and resources devoted to biologically female persons. This does a side-run around the Equality Act.
If you are a radfem, this is identical to saying "men are demanding access to women's areas."
It seems that most organisations are taking the view that transwomen are women to the extent that they are overruling or dismissing the right to sex segregation, and while the EA2010 allows necessary and proportion sex segretation, that's open to interpretation, and I think I know which way it will go in a court case.
My views are deeply unpopular here, so I'm not sure how much help this it.