We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
If I were you I'd get the data you want backed up in one place. If you're dual booting have a separate drive or partition that is a filesystem both OSes can safely read and write.
Then whatever OS you usually leave running overnight configure that OS to backup your single shared important data location.
If you over-complicate it, it will blow up in your face someday.
Backups and even accessing files can be a bit of a pain with two OS. NTFS drivers for Linux are a thing and they mostly work but I would not overly rely on it. For this reason, I would consider using a NAS for storing and accessing your files. It can also handle your backups with Rclone.
As it was said, the best option is to have separate storage for backups, ideally backup NAS. IT will also give additional protection and would help you to fit 3-2-1 backup rule. As for encryption, rclone should handle this. Alternatively, you may check starwinds vtl with backup software, cloud provider won't be able to access the data stored inside of the virtual tape, but you should check if it works with google drive.