It seems that installing the eink optimized "Jotas" build of CoolReader (also known as jCoolReader) dramatically improves battery life (provided you leave WiFi off). I only have three days of use now, at around 2-3 hours per day, but the battery is around 85%. I think that PROVIDED you stay within the eink optimized application, battery life should be significantly improved.
https://www.mobileread.com/forums/sho...d.php?t=268388
There is a feature in Pearl and Carta eink screens called Regal Waveform, which allows partial refreshes of the eink screen. However, it must be implemented at the software level. That means apps and the operating system needs to be coded with Real Waveform support. It seems that the Onyx team hasn't implemented Regal Waveform.