BarbieJellyBabyBrain, self-IDed gender won't stop sexists anymore than racists who subscribe to the one drop rule will let someone's self-ID as English (say) stop them.
Surely the thing to get worked up about here is the existence of people who behave badly towards others on the basis of how they categorise those people. Rather than complaining that self-ID will not cure problems no-one ever said it would.
"The thing is...if we deny all differences between women and transwomen, then surely nobody's needs get met?"
Well yes, different subgroups have different needs. BME men may have different needs than white men. Lesbians different needs than other women. I don't see calling BME men and white men "men" as being the same thing as eliminating all differences between them.
"if we acknowledge that everybody has rights but different needs then surely the outcome is better for everyone?"
Yes.