Not quite what you're asking for, but I do have SyncThing sync all my ebooks to all my devices (phone, laptop, desktop) except my eink reader, and it's very nice! I've thought about trying to get SyncThing set up on a modded Remarkable or something to complete things. As a sibling pointed out, the files are small enough it's pretty easy to have the entire library on every device, and the peace of mind & convenience for planes, etc is wonderful.