Considering all the chatter about tossing Perez for lack of performance, I think they could want Sainz Jr. if he did not also come with Sainz Sr. (and perhaps his cousin manager).
I am sure Toto is still holding out hope that Max leaves Red Bull, but even with the RB20 not being as dominant as the RB18 and RB19, it's still pretty dominant and the technical team behind him is still quite strong.
My guess is Toto would prefer Antonelli in the car, but that was predicated on Mercedes still being generally in the weeds this season and next. Considering how much they have improved (and should improve even more in the second half when they go back to the new aero package they tested in Spa), Toto may now feel that having a raw rookie in that second seat could end up costing Mercedes finishing positions and points next season and possibly beyond.
If true that Sainz has a "walk away" clause with Williams for 2025 if Mercedes extends him an offer, this likely means that Mercedes and Williams have come to a private agreement that if Sainz goes to Mercedes, Antonelli will replace him at Williams. And if Antonelli lands at Mercedes, then Williams has Sainz and is not left trying to find a replacement for Sargeant towards the end of the season when everyone else has made their driver decisions for 2025.