I saw mention that you could get your save games out via Google Takeout, but I don't know if that's verified or if it would even be in a format that's usable to someone.
It is mentioned in the article itself, it also implies that it is not useful for multiplayer saves.
Unfortunately you can't, because Red Dead (((Online))) locks your account on the platform you play, his account on Stadia will simply be erased into oblivion.