Sorry, but as a femboy enjoyer, I lost hope for that coexistence and potential of alliance after watching the mess with Bridget, the celebration that followed, and the responses that "femboy erasure is good and necessary". There were numerous less prominent examples afterwards too. After seeing all that, it's hard not to perceive transing attempts as attacks on things I like.
TBH, Bridget was always kind of an uncomfortable character to begin with. The whole bit of dressing as a girl out of a sense of obligation as opposed to their own personal choice. I did celebrate Bridget announcing they feel they're a girl, but less because I'm out to erase femboys or push trans stuff but just happy to see Bridget taking agency for their identity after being so passive about it for so long. I feel similarly about Mai Natsume from BlazBlue, who admittedly was physically female for her whole identity arc.
Anyways, I do think it's important to remember that nobody can "attack" the thing you like and there is need to "defend" it. It's important to keep perspective there -- the conflict you're concerned about mostly doesn't exist in Japanese online discourse for the most part, so any translated manga isn't going to reflect that conflict. And even if the conflict did exist, MD's forums aren't where it'd be decided anyway.
In fact I can even point you to an untranslated Pixiv series about a TS girl and a femboy that got transformed as well and how the story treats the latter as a bad thing that shouldn't have happened. Eventually the femboy regains his true form but still passionate as ever about his feminine fashion. (and remains good friends with the TS girl who is very passionate about her new form)
Also as to Onimai specifically -- the anthology manga show that this story supports a number of different views of the same characters. Meaning that Minato as a big-boobed squirming TS girl complementing Mahiro's own TS moe and Minato the femboy can both exist. I strongly suspect the next anthology volume will have at least one chapter each devoted to both.