I host a Grimmory server, and Readest’s koreader sync works really well with it.

I only read on mobile (android) using Readest but they do have desktop versions and I assume they’re just as good as the mobile version.

They just added functionality to play audio overlays (like from Storyteller), too, which is something I’ve wanted for a while.

All they know is what metadata is attached to the file. If you tag your home movies with metadata for the new Odyssey movie, you haven’t broken any laws.

I was recently looking into this myself, and I saw someone recommend FileFlows, but it was recommended for ease of setup/use, not because it’s open source. Figured I’d mention it, but note that I haven’t had a chance to check it out yet.