I think you mastered the task of getting battery duration out of you Poke 3 and nothing can be done to get significantly more battery life.
While snapdragon is way more optimized than other SoCs found in android e-ink devices, like Rockchip, is still a general purpose mobile system and will never reach the battery life of industrial SoCs, specially Freescale/NXP.
Of course you can try to squeeze the battery, but IMHO doesn't worth your time. If you still want to do so try with
while you're using your device. Look for power events/wakelocks and general errors.
Other thing you can do is replace your launcher with
Simple Ink launcher. After you do that disable (via android settings) stock launcher and check you can still access stock reader (on some onyx/likebook devices the launcher and the reader are in the same app).
I can't speak about recent Onyx, but on Likebooks there is a service running that's in charge of monitoring the storage and notifying the stock launcher/reader each time a new book is added. Disabling this would save a bit of battery life, but might made your stock app annoying.