Agreed that ao3 would probably be a better bet (not a lot of trans rep in Yaoi when it comes to MCs, and not much more when it comes to supporting characters ).
If you're looking for *smut* with that premise, I have one F-ed up recommendation where the main character isn't exactly trans ftm, but I feel like the MC's body is the equivalent? (um, magic drug swaps his dink for a vag and it never goes back)
The story itself is messed up and the couple is toxic AF, but I'll link it anyway
https://www.mangago.me/read-manga/boy_meets_girl_1/
https://www.mangago.me/home/mangalist/100751/
Hopefully you like some of these stories. It’s a start.

does any1 have good recs for yaoi where one or both of the mls are trans ftm?