I don't think there's any chance for Audi to get Sainz any more. Sauber is so poor right now, it's really sad. It will take at least until 2030 to get them anywhere near a podium and who knows how long until challenging for wins, Sainz doesn't want to waste his time waiting. It all depends on Max and Red Bull relationship, but let's be conservative and expect them to continue together even if Marko retires on his own soon.
I think Alonso retires and Sainz gets his seat with his blessing and possibly even mentoring. Being a long-term Honda works team is the best bet for him, Mercedes is a no-go and McLaren seats are taken. I don't think Sainz will want to go to another team and not receive the #1 driver deal, he certainly has the right for such expectations, so Red Bull is highly unlikely in my view.