Repealing the GRA isn't going backwards, it's going forwards.
It cannot be considered reasonable to expect women (and indeed men) to accept a law that says the fundamental difference between women and men is mental, and if one thinks "like the opposite sex" it means one might as well be considered as the opposite sex.
We need to recognise it's a law that was formed in sexist, homophobic times. It may have made sense, even been humane, in that context but society has moved on. The GRA today is an anachronism that far from being enlightened just encodes sexist attitudes and we no longer need it.
Any compromise that tries to square the circle of a legal sex that is not ones real sex is ultimately going to fail. You can't legislate reality into something else. It's like trying to pass a law to adjust gravity. You can say it's true, you can pretend to see it working, but any time sex actually matters the construct is going to fail because reality hasn't changed.
The kindest way forward IMO is to disconnect gender from sex altogether (which after all is simply recognising the reality that whatever trans people are feeling, it is certainly not being the opposite sex), and treat it as a thing in its own right, another axis of commonality between people akin to religion or regional identity but self-identified.
Give it its own language and let people lobby and organise whatever new rights, spaces, celebrations or protections they feel their gender group requires.