Babies aren't identical, full stop. Even identical twins have different personalities. I had m/f twins, now adult, and raised them as gender-neutrally as I could, just watching their interests develop and letting them play as they chose with everything. We used a toy library and they could choose. Both grew up to be creative - one is musical, the other artistic. Their interests showed early. Both had dolls, both had bikes etc.
I think an awful lot of what we hear called gender is just personality and interests. Perhaps not all, but probably most.
If society weren't so obsessed with labelling interests and activities as being for girls/women or for boys/men and judging people for breaching imaginary boundaries, a lot of gender dysphoria would subside or be much more manageable.
If we didn't fuss so much about what people are 'supposed' to wear - especially boys and men - happiness could be wider spread. After all, women can get away with wearing practically anything and nobody bats an eyelid. Who wouldn't want that freedom?
Sex, though, that's immutable. You can put yourself through painful surgeries or take cross-sex hormones for life and every single cell in your body will still announce the XX or XY (or XXX or XYY or whatever if things went a bit awry) that was your inheritance when sperm met egg. That's just fact.
Wear what you fancy, live who you love, follow your interests, call yourself whatever you choose, express your individuality, I wouldn't dream of interfering or criticising.
But if you've had a male puberty, it's not fair to colonise women's sport. And if you've had male privilege in your work life, it's not fair to sweep in and snaffle awards intended for women who didn't And if you have a penis and are older than 5 or 6, it's not fair to colonise the girls' or women's changing rooms. And if you are a sex offender or rapist, it's not fair to self-identity into a women's prison and skew government crime statistics so it looks as if women suddenly started raping people.
Live and let live and respect for all - and if you're a man and use the ladies and behave like a lady there, I'm good with that, too. It's all the other bog standard men who will follow you in if you insist on self-ID that really worry me. I realise I'm an outlier here.
No boys in girls' toilets at school though. Girls learning to manage periods do not need boys hanging around outside. After 18 maybe.