Going out on a limb, but assuming geneder dysphoria is the result of genetics or something seen prior to birth, it’s logical to assume that Federation Medical Science would actually...loaded word time, but not meant negatively here, purely scientific...’correct’ it before birth or immediately afterwards to ensure that the difficulties it presents the individual would not occur. It’s always a match, every time. Alternatively, given the approach to Eugenics, and most other forms of transhumanism in Trek, it’s entirely possible that any ‘tinkering’ is effectively banned. Combine that with there basically being no differentiation between gender roles, and greater control over reproductive biology we see in Trek thus far, and it seems it may simply be irrelevant due to either tech (my first example, instant transporter sex changes, surgical advances that make it basically impossible to identify gender reassignment as having occurred) or societal (there is no gender difference in roles or opportunities, the concept of ‘male’ And ‘female’ in humans now exist purely as biological difference, there are no other differences, thereby lessening the occurrence) changes making it that way. It’s entirely possible that you can pick any given crew member we have not seen in childhood or as a baby and say ‘there is a trans character’. We know that editing of genes and what have you is performed before birth, and we examine the hornets nest with B’Ellana and Tom and Miral. But this all just looking at stuff.
In terms of nationalities on DSC...yup...it’s full of more Americans than any other series of Trek. On the other hand, it’s not as much of an ensemble cast, so we don’t know who else on that crew hails from elsewhere. It is a bit clumsy though.
In terms of ‘seeing people like yourself for so long’ well, that all depends how much of your individuality you identify with. The closest characters to myself I have seen in Trek all have massive identity differences that would theoretically disallow me from identifying with them...If I allow myself to be limited by gender, skin colour, ethnicity, nationality. Fortunately, I tend not to.
Representation is tricky thing, once you start slicing it thinner and thinner, or trying to please disparate groups that make a larger group...look at Chakotay, every Native People and none of them.
Even Doctor Who, seen as a bastion of progressiveness for many many years...cocked the Trans issue right up, by having their trans actor play some kind of mutant subnormal...albeit one with a touching story.
It’s a tricky issue, and I don’t think there is a ‘right’ answer for Trek, narratively at least.
For a more..nuanced...look at it, or to find some representation as far as Trans goes, I recommend Knights of Sidonia.
Me? Well...Marina Sirtis is as close as my corner of the world got to representation, Ok maybe O’Brien. But I don’t have the same cup size as either. Ok. Maybe O’Brien. I see more of myself in Geordi, Bashir, Kira, Paris. Which sure frustrates the argument, because as I said...a certain way of thinking would not allow that way of thinking.
That said...it wouldn’t hurt for the show to try a little harder for those people who do not share that approach. There’s no harm in it at this point really, is there?