converting all facilities to unisex is arguably the fairest and most sensible option.
Apart from the ethical issue of - what about fairness for women? and for that matter, men, who also value their privacy and comfort? - have you thought through the practicalities of your suggestion?
First of all, building regs in England make single sex facilities mandatory, unless there isn't enough room in a building.
Secondly, workplace regulations also mandate separate single sex facilities, again with the exception of space.
Thirdly, mixed sex toilets have to meet stringent building regs specs, you can't just 'convert' existing single sex toilets by changing the signs on the doors.
Fifthly, who is going to pay for this massive building project of removing existing single sex toilet blocks - because they will not meet building regs as mixed sex toilets - and building brand new, building regs compliant 'universal' toilets, in sufficient numbers to match the 'throughput' of the old single-sex facilities?
Sixthly [is that even a word?] this massive building project of new mixed sex toilets is going to have to be in addition to the required provision of separate single sex toilets in new builds, as well as adapted toilets for disabled people - even in a large building, that's a heck of a lot of space taken up by a heck of a lot of toilets!
Seventhly there are the health and safety issues caused by the floor-to-ceiling design of mixed sex toilets, where medical emergencies are undetectable from outside.
The simplest solution is the obvious one, which has worked well up to very recently, when a small but noisy and sometimes violent group of men and their allies started demanding to use the women's toilets:
biological men use the men's, biological women use the women's, people with disabilities who actually need the adaptations use the accessible toilets.
Your 'fair and sensible' suggestion is neither.