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.

It does, and it supports Linux and macOS too

Readest (https://readest.com/) can also use KOReader sync and works on iOS, Windows, and on the web.