Colours are considered gender-coded, so you could make the venus and mars symbols into masks that change colour based on gender identity, i.e. blue vs. purple vs. pink, but the actual venus or mars symbols are tied to what parts a character has? I feel like it could maybe work if there was an intuitive way of changing the two in the menu, i.e. you have "Feminine" "Non-binary" and "Masculine" in a list next to the symbol, and the colour of the symbol changes as you hover over each of the identity options. As for the actual venus vs mars symbols, you could make it a blank circle with a hint to "spin to change character parts," and as they spin the circle it changes into either a venus or mars symbol while the parts change. Could also just make a secondary symbol that's visible to you at all times that shows whichever symbol you set it to, regardless of your character's parts. Kinda like a WoW transmog (I hate that that's the only comparison I could think of but here we are.) IDK if any of what I just said made sense.
I will probably design specific icons to represent biological functions: sperm on one side and egg on the other. I'd like to avoid any gender based symbolism since most of them are hetero coded UwU Plus it's kinda cool sometimes to come up with fun new icons XD but thanks for your ideas it's a very interesting topic indeed!!