totally agree with you, jenny.
i wanna expand on what youve said, and what ive said, for further clarity for OP and anyone reading. i think theres a balance between 'make this character all about x identity' and 'make this character be x identity but never mention it.' for example, in your story Keikos sexuality is still plot-relevant despite not being the centre of the plot. i think it being irrelevant would be unrealistic.
the reason research is so important is that important, non-tokenising representation needs to be realistic. both making a character 100% one aspect of their identity and making that aspect of their identity an irrelevant Fun Fact a la dumbledore is tokenisation imo.
OP, for a trans character, this can be as simple as, say, dropping mentions of gender clinic appointments or acknowledgement that taking hormones is part of their daily routine. or it could be as complex as a subplot about, say, them fighting for bathroom rights or an arc with a parent that involves their gender - that kinda thing requires more care, but isnt off limits if thats something youd want to do. id avoid acknowledging their transness through things like them being misgendered or outed to someone, though - focusing on the negatives sends a depressing message to trans audiences, rather than the uplifting one you want to send.
the sisters webcomic here on tapas has a trans side character, and i think they do this pretty well, although they lean more into the hardship side of things than you might need to (not a criticism, just an observation. it suits how their comic is ig) the character is a trans woman who lives with her religious grandma, so she has to recloset herself when at home, but is accepted without any fuss as a woman around the rest of the cast.