It is possible to get Syncthing on PocketBooks. I've got it running on a PB700K3, but it should work on older devices. Here are some links for more information:
mikedigriz/Syncthing-for-PocketBook (good information, but a translator is needed)
syncthing-on-pocketbook (a bit dated)
https://forum.syncthing.net/t/pls-re...etbook/21370/3
Once it's setup and working, the only issue is that the PB needs to be restarted for new books to actually display in the library.