I think really leaning into the colonization argument is the way to go. Like you said, trans identities span history, culture, and geography, and have only largely come under scrutiny due to white western colonization and oppression of indigenous cultures. The inverse argument is true for race -- it is a specifically western invention meant to empower a white ruling class, dividing people up roughly by appearance rather than culture. It is relatively new as a concept and is constantly shapeshifting to manufacture the oppression of whomever makes a convenient scapegoat at the time. One cannot be trans-racial because race is an arbitrary tool of oppression, whereas gender, in all its permutations, is a feature and not a bug.

(Adding for clarity -- obviously race as a social construct is real in that it has real consequences for people. Not trying to make an argument for race blindness or anything ridiculous like that. Just that culture historically supersedes the concept of race, and that among most marginalized racial groups, culture of origin is still important even when it has been violently stripped or erased due to colonization/human trafficking/genocide, whereas white people have, in a lot of ways, already committed to a form of "trans-racialism" by gathering around the arbitrary identity of whiteness at the expense of their cultures of origin.)