gender dysphoria is not what I'm talking about, since not all transgender people have dysphoria

right, I get that, but most research has not treated being trans outside the context of gender dysphoria, so when talking about studies or the clinical and scientific context usually non-binary and non-dysphoric trans folks are left out of the picture.

It is good to be expansive in our concept of being trans for social justice reasons, and to not gatekeep for harm reduction reasons, but since scientists and clinicians are gatekeepers, what we can say about their findings are limited to the criteria they use. It is usually more accurate to say "a study found X or Y about people with gender dysphoria", even though it's not uncommon for that to be presented as "a study found X or Y about trans people" in more mainstream contexts.

To be clear -- "transgender" the noun is not referring to a person ("that person is a transgender"* -- proscribed) but rather as a substitute for "transgenderism"* (proscribed).

I have never seen this usage, and like you I'm skeptical that is right. I could see "being transgender" as a substitute for "transgenderism", but not just "transgender".

but it's also clearly not purely genetic, given there are identical twins where one is cis and the other not.

The presence of identical twins where one is cis and one is trans is not proof that gender identity is not genetic - there are many reasons people do not transition or acknowledge their gender identity, such as the strong social pressure to not be trans. There can also be epigenetic differences so while identical twins may share a genome, how it is expressed differs based on a variety of conditions that alter epigenetics, such as stress or illness.

We see the same with sexual orientation by the way.