Does OPDS support sending to the server the current position in a book? That's basically the only feature I would need a server for. Books are synced with Syncthing, no need for anything fancier than that. Of course the ebook reader must be able to send the position to the server when in the same network.

I'm also almost always in 4G coverage but there are planes, trains, mountains, tents in the middle of nowhere, etc. Apps that work offline are a safer choice.