We may be talking at cross purposes.
I think race has been constructed by racists and slavery, and has been a disaster, but ethnicity has a basis in people's lived experiences which are ultimately rooted in material reality. We would lose most of the world's knowledge of environments, medicine and so on if there was no such thing as ethnicity. It wouldn't be possible for people to survive in different places without developing unique cultures, so they are here with good reason. In fact, the construction of race is connected to material reality - grabs of land and people, but it isn't a useful category of people for any positive reason other than as a reaction to ending the consequences of that and helping people understand what has happened.
With biological sex, I am arguing against the idea (I think!) that there was biology, on which people built negative social constructions exist and which have negative social consequences for the group now socially called women.
I am arguing that a material reality exists, which we comprehend through a social construction called biology (and other social constructions) and that construction itself is prone to issues of social power. Materially real females are then harmed through social constructions and material events which are maintained through power which is both constructed and real, and the consequences of that are materially real and socially constructed.
FGM, for example - done to the genitals (really female, named and understood through constructs), demanded by a whole society with a proposed reason (social construction), done through power (social, who is named as controlling resources they may withold) and by literal power (holding someone down and harming them, with a social consequence (status of females) and a materially real one (illness, infection, impairment, death).
There isn't a point at which you can separate the two concepts, other than to say that some aspects of material reality will always exist, while some aspects of material reality and all social constructions can be changed.