The latest version of KOReader is simply fantastic for the Mars. The way I installed it followed advice from somewhere else in this forum (sorry I don't have the reference):
- Install KOReader from the Boyue app store.
- Install the latest KOReader from koreader.rocks for ARM Android (i.e. the ARM .apk)
IMO, KOReader is both the most faithful and most flexible ePub renderer, and the interface has gotten much less "clunky" since 2015, for sure.