Yeah, this is why I said my homework increased
Ah, I thought you meant my homework increased. That makes way more sense now.
There are several that I linked it the Github list, including kirikiroid2 on the very top.
Because it said it was for playing visual novels it didn't occur to me that it could also be meant to read normal books as well. I guess that's a benefit of a format that supports so many features though!
What does koreader use to synchronize bookmarks across devices?
It checks the file hash or file name (depending on settings) and syncs your progress across devices that have matching files using a centralized server. There is also an option to self-host the server. Beyond that I don't know the details, but it's open-source so it shouldn't be too difficult to figure out whatever other details you're curious about.
Thanks for all your work on open-source reading technology!