if a transwoman has fully medically transitioned, she is going to be at risk in the men's toilets. Does that mean there should be a third space? How would that be implemented?
With the greatest of respect, why are you posing those questions here rather than to the many many charities, groups and lobbying bodies that exist to support trans people? Surely these are exactly the type of challenges and needs of trans people that such bodies exist to solve?
is there a level of medical transition that should entitle a transwoman to use women's toilets?
No.
Firstly because dressing up as a thing does not make you the thing. Women's resources and protections are for women. Womanhood is not like a Panini sticker album where you just need to collect enough parts to complete it. That is incredibly reductive and offensive to women.
And secondly, if a man were to make changes to himself that make him equally vulnerable that aren't a facsimile of womahood, would you suggest women's protections should include him as well? No. So this argument is not in fact about protecting the poor man, it's still all about his identity as a "woman". If he hadn't decided in his head he was a woman and started to use that name for himself, no-one would even be considering whether women's spaces are appropriate for him.
From the point of view of women, this is like someone stealing your car and then instead of you getting it back, you are being asked in all seriousness how much use of your car you think it's fair to let them have because after all, they do need to get to work.
Also, finally, for the millionth time - for women this is not about toilets.
I get that Genderists are obsessed with toilets and the belief that trans people (well, really trans women let's face it) absolutely have to be allowed to pee into the same bit of ceramic as female people or their lives become unbearable, but for women, it's far far more than this.
It is literally our right to have our existence as the female half of the species recognised in law, society and culture. It is our right not be reduced from simply people of female body with full diversity of minds and capabilities, into people who are assumed to have a womanny frame of mind that we share with each other and a few male people as well. It is our right to say "our bodies have consequences and we have a right to talk about those consequences and to politicise and activate to make things better for ourselves"