The argument from biological sex stands just fine on its own. That's why, over the past few years, the TRA position has deteriorated into a miserable pile of well-poisoning, ad homs, and appeals to improper authority.
Also, the argument from biological sex renders moot almost all concerns and arguments about gender expression.
Gender expression is already protected from discrimination. Beyond that, it doesn't mean anything and doesn't need to mean anything, for the purposes of trans rights. A dude in a dress has all the same rights as a dude in trousers. A dude who makes a big production of how feminine he is day to day has all the same rights as a dude who makes machismo his entire personality. A dude who makes a big production about whether he's a dude, a lady, all of the above, or none of the above, on a day to day basis has all the same rights no matter how he slices it.
So long as we all agree that biological sex is real, and that certain things need to be segregated by sex, gender is irrelevant.
The problem is we don't all agree. Ironically, the root of the disagreement is the irrelevancy of gender. For all practical purposes, the question of trans rights is a question of the right to transcend sex segregation. The right to transcend gender norms and stereotypes is already well established.