For what it's worth, you are supposed to be able to access documents stored in OneDrive in Linux. (I haven't tried it myself; I'm just aware of it.)

Basically: Go to GNOME's settings, then select online accounts, and sign into OneDrive.

Here the same instructions with lots of details and screenshots: https://www.omgubuntu.co.uk/2024/04/set-up-onedrive-file-access-in-ubuntu (Nothing in that link is Ubuntu specific. This applies to other distros too, like Fedora, Debian, Arch, Mint, etc. — as long as you have GNOME 46.)

Other desktops have to use various other options, but aren't natively integrated: https://linuxstans.com/microsoft-onedrive-on-linux/

Of course, once you have access to files like this, you'll probably want to copy them locally and back them up yourself.

(I suspect Microsoft was trying to make it so people don't lose their files, ironically, by handling document storage themselves? It's still absurd that they do this so aggressively.)

My favorite backup software is Pika. It's powered by Borg, has a friendly UI, and supports both local (including USB drives) and remote locations. https://flathub.org/apps/org.gnome.World.PikaBackup

Deja Dup is also good and uses Duplicity behind the scenes. https://flathub.org/apps/org.gnome.DejaDup

There are a ton of other backup solutions too, including Vorta (another UI for Borg) and a bunch of command line tools, like rclone, rsync, etc.