Sex though is a category that we use for animals, it's a scientific term with medical usefulness. While it doesn't line up perfectly with gender, and doesn't always match up secondary sex characteristics with chromosomes, for 99% of people it does. And medically there is usefulness to "sex" as a heuristic, again for 99% of people. Biologically "male" people usually have higher instances of certain diseases, even if they have transitioned. So I think as a category it shouldn't be done away with. But it doesn't have that much societal usefulness. It's more a medical and scientific term. For most societal contexts, gender can be used instead of sex.

However I also do think that in certain feminist contexts talking about the biological "female" is also important, because while not all women are female, sexism historically was based on reproductive control, and many things that "women" experience now in terms of sexism, whether or not they are cis or trans or whether they are fertile or infertile, they still experience the legacy of a patriarchy that wanted to control female reproduction. It's like how while not all black people in America are descendants of slaves, the legacy of racism is tied to the slave trade. So while I wouldn't use female biology as an excuse to exclude trans women from feminism, I do think that female biology shouldn't be excluded from the discussion of feminism because it really did contribute to how women historically were oppressed and still are today, even if patriarchy now oppresses all women, whether or not they are all "female". Does that make sense? It's a nuanced topic.