If there is a legitimate case of a person who is transrace.....I can't conceive of it as of yet. My initial reaction that it would be one the scenarios you outlined. Either they have an ethnic romanticism type of thing going on.....or they want to lay claim to a burden that they don't actually experience (victim-hood seeking)....or they feel that their voice will carry more weight/taken more seriously....or they want access to a tangible benefit that they don't deserve (race-relevant scholarship/government tax benefit/etc). Most likely some combination of all four and none of them are legitimate or just.

And just like you mentioned earlier.....those possible motivations are not as present for gender transitions. Yes, it's true that trans issues also have scenarios that might involve opportunism (males transitioning to females and then competing in a woman's sports league)....but those instances are EXTREMELY rare.