These are all good points, but they at the moment will add a lot of fuel to the fire of it's all too complicated, too expensive, requires rebuild from the ground up and the best thing to do is kick it in the long grass with a lot of consultation while men crack on using women's toilets.
The goal is to implement the SCJ immediately and give women back equal access, which requires men out of their spaces.
No one is going to treat men as they did women, and just say 'go in ones you're very unhappy about or go without' - apparently people with ovaries are a fuckton more resilient and capable than those with balls.
There has to immediately be a cheap, accessible, practical alternative to sex based provision. This is most likely in the short term to be enclosed single use cubicles and most likely to be the commandeered disabled ones, with all the issues involved, or re labelling of a proportionate number of cubicle rooms as gender neutral. This immediately returns women's single sex spaces back to accessible ones so that all women have somewhere they can go, and starts work on the idea for men that however they feel, they can't have that one, they have a choice of two others.
Yes men will kick off about this and find a whole raft of excuses about ghettos and othering and outing and safety and anything else they can think of, because they want to be where the non consenting undressed women are, and they do not want anything in the world that makes clear they are not actual women. Unfortunately as women have rights too, that is tough. The middle ground and compromise is to provide accessible facilities alongside single sex facilities, and those who consent to use them can do so. If they choose not to use them at that point, it is a personal decision.
It is not going to be perfect, and won't be probably for years, but the priority has to be to start with, getting men out of women's single sex spaces and women having rights being once more an accepted thing in the UK. I worry that making it complicated now plays into the hands of those invested in ensuring that the SCJ never makes it into reality.