You might be better off with an e-reader running full Android. I use Syncthing Fork, and it lets me do two-way sync between my tablet and a Macbook, which acts as a local server. No middleman cloud service required.
If you run syncthing on android you lose its power management. Also it's a horrible OS for long running services for other reasons, one of the largest being how huge apps are and how it quickly kills things when memory gets low.
I am using Syncthing Fork from F-droid, and you can configure it to respect Android's power saving features. Besides, you can turn sync off entirely if you choose.