That's the point. I don't want a suitable replacement, and I don't care if it's circular because my goal is to bring society closer to gender abolition. By making gender circularly defined, we are eliminating any pragmatic use of it in a social context. I don't want gender in the dictionary. Besides, just because a word is defined a certain way in the dictionary today doesn't mean that we must adhere to the dictionary tomorrow. Words are not immutable.

Masculinity & femininity are the genders

"Masculinity" and "femininity" are not genders. "Masculinity" and "femininity" are a part of the genders "man" and "woman"; they are not separate entities. If femininity is a gender, should Jeffrey Star use the women's restroom?

"Man & woman" in humans is no different from "billy & nanny"

Trying to say the terms "billy" and "nanny" in goats are comparable to the terms "man" and "woman" in humans is ignoring the social context of why we have these words in the first place. They weren't just extra words we created for fun, they're entirely constructed for the purpose of pushing the narrative that a male must adopt the roles of a man and a female must adopt the roles of a woman. Goats don't have the cognitive capability to oppress each other on the basis of their sex, nor did they invent the words "billy" or "nanny" themselves.

Binary transgenderism supports the view that gender is innate, embracing it sounds like the opposite of gender abolition. If gender abolition is the end goal then non-binary, specifically agender is the way to get there.

Well, it certainly "sounds" like the opposite of gender abolition when you strawman what it means to be a binary trans person. Just because you're "binary" doesn't mean you are "conforming" to a form of masculinity or femininity, it means your gender is one of the two genders established in our culture (where gender is whatever you identify as). I like video games and wearing t-shirts/jeans. Does that mean that I'm not a binary trans woman?

I'm advocating for a world where gender doesn't exist, or at the very least, doesn't matter. This can only be done from a pragmatic perspective. In our current society, for better or for worse, people cling to the terms "man" and "woman" because we have been socialized to accept that having gender is the only way we can coexist. The acceptance of binary trans people would directly lead to the acceptance of non-binary trans people, and then eventually the elimination of gender due to its unpractical usage ("anyone who identifies"). I want people identifying as ketchup and mustard genders one day in the future, if not a full abolition of gender. To me, a useless circular definition for the greater good of society is always and improvement from a sexist, concrete definition.