I use Syncthing on all my endpoints Windows and Linux (can't speak for Mac) to sync to my TrueNAS server. It has a built in tool to just back up to backblaze on a certain schedule.
I know you can use Syncthing with unraid in Docker. I have it set up so sync all endpoints to my server and then the server pushes the latest changes back to all the endpoints. This is overly redundant and you don't have to do it that way but all endpoints and my server would have to die at the same time before I lost any data. It's sort of a backup scheme in and on itself.
I don't know about SteamOS specifically but you can dual boot windows and Linux on the same drive. Each can exist on their own. The partition will still show up in windows but you can ignore it and it won't do anything. The problem is Windows and GRUB for Linux. Windows has a bad habit of just bulldozing GRUB and making Linux unbootable.