even “biological sex” is pretty complex (In science, I have heard that the “three G’s” (Gonads, genetics, genitals) model is the standard definition, but scientists who research biological sex seem to consider this an extreme oversimplification)

Yupp. That’s my understanding. I found this video by a biologist enlightening in this matter: Sex and Sensibility by Forrest Valkai (1:40:26, Youtube)