Do you actually think that a woman's prison is "a place for potential rape victims, regardless of sex"?
Women are real people. We aren't a secondary category of sub-human that exists for the benefit of men.
There is no surgical procedure that can make a man into a woman. A eunuch is a eunuch. An artifical blind fistula is not a vagina. There is no such thing as "full reassignment surgery" except within a brainspace that says that a woman is any body that is configured to be used as a thing for a man to ejaculate into which is a horrifically sexist and patriarchal idea of womanhood which we reject totally.
A man who has had his genitals removed is just as much still a man as a man who has had an arm or leg removed, which is still just as much a man as a man who is fully intact. The idea that losing that little bit of flesh stops someone being a man is part of toxic masculinity that belongs to last century.
Male prisons should have proper safeguarding in place so that all vulnerable prisoners, which will include transwomen and plenty of other men who have other reasons for being vulnerable, are protected.
"Not being raped with a penis" is far too low a bar for women to accept. A man without a penis is still an invasion of dignity, a source of threat, of feeling used as a prop for someone else's fettish without consent, potentially a dominating physical presence, with muscular strength still likely to be greater than the average woman, and entirely socialised to dominate women and put their own needs first above women's needs. A very large proportion of trans-identifying male prisoners are sex offenders and it is well known that rapists often do it for the power and domination, rather than for the direct sexual stimulation, and will commit the penetration with other objects if they can't use their penis (e.g. if they can't get an erection on-demand) so having had their penis removed is not going to stop an abuser from being abusive.