i don’t think there’s anything inherently wrong with having gender in mind from the jump, or coming up with it later, or deciding to switch a character’s gender during the writing process. With the last one, I think it really comes down to execution and your reasoning.
sometimes i change a character’s gender because it just strikes me while writing: ‘actually. no. __ makes more sense to me as a boy/girl/non-binary person’. Our characters are our creations, I’m not sure there’s always a way to explain why they change in our minds. the non-b character i mentioned in my last post was originally a boy (first came up with them in 2005 at the age of 14) and a genderfluid character I’m writing was originally a girl (came up with them the same year). both of them have changed in many ways since i originally came up with them and their gender change was actually relatively recent, but it was never because ‘well, my story is about x, so __ should be __ to tie into that’.
Themes should tie into settings, I think, and I’d much rather create a new character that the setting or environment might naturally spit out than change a pre-existing character to fit a theme. if that makes sense…?