Sneakernet crew here too. My work offsite backup is in my backpack. Few times per week I do a sync which takes a few minutes and take it home again. (The sync archives old versions of files and the drive is encrypted.)
We tried several cloud-based solutions and they were all rather expensive or just plain hard to run to completion or both.
If you find yourself in Ottawa, Canada, check out the Diefenbunker, built by the government of PM John Diefenbaker during the Cold War
It does, and it supports Linux and macOS too
Readest (readest.com) can also use KOReader sync and works on iOS, Windows, and on the web.
Perhaps this will help