In the original JP, does konpeito ever use significantly gendered terms to refer to themself? as opposed to ones that are accepted as being used by both (i.e. boku)
hat konpeito is much less cute than rabbit ears konpeito. but it seems to be more about speech patterns to me, but maybe the face shape changes a bit too?
they use gendered terms mostly in accordance with their presentation.
Atashi feminine, boku male (as you say it's accepted for both male and female, but their general speech patterns get very masculine here as well), but also with a lot of un-gendered referents in their feminine form. Tsuzuki also refers to them in an ungendered way, though mushiro is the only one to actually use a gendered pronoun for koneptio (and only once, early on) so that's not really indicative of anything. As such, it ends up mostly just being a judgement call on my end.
Zatto also uses boku for instance, but that's more reflective of her bluntness than of any gender expression in my eyes.
Regarding their different mannerisms/appearance, there's a good bit different between the two- especially with regard to hands/feet. I don't know if you've noticed, but Konpeito never wears shoes; in masc form, those dogs are a hell of a lot larger.
I personally find hat peito to be quite smoking hot, though I understand what you mean about mannerisms. They're a lot more assertive here (which is part of the masculine persona, but also I believe part of them just being more content with their life and feeling like they have a place to belong).
Someone else made a note about the masculinity representing a sureness in their life, and I think that's an interesting way to look at it (that is arguably somewhat corroborated later on!). I don't know if I necessarily agree, but it's certainly interesting.
I guess a lot of it is I can instantly notice a whole ton of things different between the two forms, but only because I spend so much time looking at peito (and of course, all the other characters in this here mango).