An insightful video assessing the origins of our modern western genders and why defining womanhood is such a contentious dispute. (this video is over 40 minutes long but it was so interesting I didn't even notice)
Just finished this. I'm a bit biased (I've heard a lot of the points she's made already), but I do agree that keeping the definition of "woman" nebulous takes away its usage as a medium of power projection. It's pretty telling that most terfs are white and at least financially comfortable wheras most unprivileged people with gendernormative views trend toward the reactionary aspect rather than preservation of an institution.
I've seen a some nonbinary people try to redefine gender in terms of fuzzy blobs of tendencies so this would lend some credence to the idea that more people can have degrees of nonbinariness rather than having some arbitrary dividing lines that make enbies feel like a third gender.
I agree that the best definition for gender should be fuzzy and subjective. I think the biggest thing holding people back from abandoning rigid definitions is our rigid society that demands a nuclear family where women are baby factories and men are wage laborers. Because if that's what defines gender then it makes sense why people conflate gender with genitals and why they oppose trans people on a conceptual level.