If he truly a signed a deal for a solo, it will get done. It's not worth trying to guess how available he is. Besides, Cavill doesn't work that much that he can't film Superman movies. I think he only has only one film that he'll be filming soon so right after that, he'll likely do Superman. Like someone said, the Rock works for than anyone and was able to get Black Adam done in 3 months. Cavill has more than enough time to start hitting Superman movies.
I think they'll keep Sasha as Supergirl but it is clear that Zaslav and the Discovery want Superman himself to be a bigger part of future movies. The previous plans were by the previous heads and aren't worth being beholden to. Affleck looks like he is done with DC based on what we've heard from the Flash leaks. It is going to be Keaton as the DCEU Bruce while Pattinson plays young Batman in his own universe. Cavill will likely be DCEU Superman and get the solo movies he's been wanting