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.

I use KOReader on my Android phone, laptop, and Kobo. It’s pretty great. You can sync reading position between the various devices and download books from e.g. Dropbox and Webdav. Available through FDroid and their web site, not Google Play.