In this debate, I'm reminded of the shift to gay marriage.
Back when civil partnerships were introduced, it was somewhat of a compromise between those who wanted full equality and people concerned about the effects of gay marriage. Within the debate, prejudiced views were mixed in with genuine concerns about religious rights, tradition, and legal loopholes.
Civil partnerships worked in showing that the vast majority (all?)of the imagined problems and loopholes created by gay marriage did not actually occur once such a system was in place. This allowed people to argue for full marriage equality with real evidence that gay marriage wasn't a problem for those with genuine concerns, weeding out those whose objections were just based on prejudice.
My somewhat round-about argument is that with regards to fluid gender recognition and access to women's only spaces for transwomen who are not or are not yet physically transitioning, maybe we need that first step of civil partnership equivalent legislation, that would offer some sort of compromise. If fears about sexual predators abusing the law to gain access to women's spaces are true, then this would emerge and hopefully we'd find ways to combat it. If in practice (as I'd think likely) such problems don't emerge, then the issues can be revisited down the line with real world evidence rather than the hypotheticals we're dealing with now.