[1] https://f-droid.org/en/packages/org.koreader.launcher.fdroid...
Once your used to it, the controls are a lot better than stock kindle as well imo.
If you want a cross platform option that's pretty good across Android/iOS/Mac/Windows/Linux, Readest[1] Is another great option. It supports syncing a library of ebooks and is licensed under the AGPL[2].
[1]: https://readest.com/
The only options are stripping the DRM from kobo books or piracy, last I checked.