It is worth considering the origins of the claim that transwomen are isolated / threatened / rejected in male spaces...
this doesn't appear to be a claim with any credible or evidential argument - it is a claim being put forward as a justification for men (transwomen) to be allowed access to women's spaces.
If the claim is not credible / has no evidence-based origin, then the answer is simple - men use male spaces - so there is a simple answer to the question 'where does a transwoman pee' - answer: with ll the other men in the gents.
there are many minorities who might claim to have had a hard time - due to disabilities / race / sexual orientation / etc. however we don't build third / fourth / fifth / etc. spaces for them - to have loos or changing rooms for every minority becomes silly very quickly - so why would you for transwomen?
To build separate loos for transwomen would be to go against the SC ruling - loos are divided on a sex basis and the SC ruling has confirmed the biological facts we all knew (and which most of us acknowledged) - that there are only two sexes: male / female.
disabled loos / family rooms / loos with changing facilities for babies are not built to deal with minorities who identify differently, but because there are groups who have different physical needs - disabled people might need the frame around the loo / bigger door handles / different taps / alarm call buttons / etc. families might find it easier to have the space needed when you take small children into the loo with you, those with babies need a changing table... a transwoman / transman has no physical needs as a result of their decision to 'transition' outside the normal needs of those sharing their biological sex.
there is no need to have 'third-spaces' to deal with any of this - we already have sufficient facilities to meet the needs of our population, and there is no need to change the definition of a man (which is sex based) - the societal understanding of what a man is has always been fluid (from high heels and makeup in regency times to occasionally wearing a dress in modern times) none of this changes what a man is - and how a man chooses to appear is up to them, there are no specific boundaries...
so, there is no need for change, and we need to remember the calls for change are just another cry for validation from those who inaccurately believe that a man can become a woman.