Koreader interface with syncthing is not that user friendly to non techs, very powerful can do anything but i don't think i would advise that to non techie

Episteme is pretty good

https://github.com/Aryan-Raj3112/episteme

Just to add, KOReader might not look super use friendly at first, but I highly recommend giving it a try. It’s an amazing reader.

Once your used to it, the controls are a lot better than stock kindle as well imo.

To add to this, since the original comment mentioned kindle, you can jailbreak many of the models and install KOReader in them if you want to keep using the hardware.

Using readera with network connection disabled. Super app.