I wouldn't necessarily consider boku proof of gender; there are bokugirls, and even oregirls. it could just be the pronoun that they're used to using for themselves. i think there's enough to support a trans reading of Kei, though not necessarily enough to discern if that's the intent. it's hard to tell, because, as mentioned in this thread, how gender is discussed in japan isn't really the same as in the west (for example, the title of the autobiographical "The Bride was a Boy" would actually be considered potentially offensive through a purely western lens of transgender terminology).
all that said, i'm not holding out hope of this being a trans series, because everything about it so far screams "trap series".