Oda really doesn't want strong women on the crew anyway. Had to make us think she was male or wanted to be male even though those idiots don't realise, there's a Japanese tradition/trope of women being written using masculine pronouns or something like that in Japanese works.
They are not trans unlike Kiku, Kiku is trans and wants to be another gender specifically. She says so herself.
https://tvtropes.org/pmwiki/pmwiki.php/Main/Bokukko
At least Oda's sparing us the nonsense over her gender though thankfully.
Yamato's a woman regardless if anyone likes it or not. Oda is way too horny and sexist to have a beautiful, strong woman be in the crew and not have her be strong without simping for men and being confused for one in Yamatos case too.
Before anyone gets on my case too, I have considered myself trans/non-binary/genderfluid in my own personal inner conflicts/struggles/confusion and even discussed it with my therapist, so keep that in mind please.
If Yamato was meant to be trans, Oda would have made it more blatant as nuance isn't really his thing after all. He loves LOUD, PROUD, BRASH AND CLASH and whatever! He does have nuance but its not really that abundant either.