My stance is surgery has no material effect on their sex whatsoever and spaces should be divided by sex.
Norther your post was interesting, but it makes me think of how theory and practice conflict. So in theory, of course sex segregation is the only kind of segregation that has any point, for the reasons you’ve mentioned.
In practice, though, enforcing this completely would a) be impossible; and b) if it somehow could be enforced, mean people like Paris Lees, who look very much like any other woman and quite possibly have had bottom surgery, are in changing rooms etc with men. And trans men like Fox Fisher, who again looks like any other man, would be in the women’s facilities. Which I think would make many women very uncomfortable too, not to mention Fox himself.
I am not sure of my own thoughts, by the way. I am sort of trying to work them out by typing them as they come to me - as you can perhaps tell.