Thank you all for the responses!
The firmware version 3.0.2 was very buggy. The screen and GUI was very slow. Fortunately, I managed to update my Mimas to the 3.1.1 by simply using a .zip provided here at the forum. Named it to "update.zip" and put inside the internal storage. Browsed the file from the settings and did the update.
Everything was very smooth after the 3.1.1 update. In fact, the device became more reliable than it was at the 2.x firmware. There is no fault of it being based on Android 8 instead of the 6. Then, there is no reason to downgrade my device anymore.
Battery life appears to have improved too, I read about 2 hours per day and there is 61% of battery charge by now, 8 days after full wipe and full charge.
For anyone in the need to full wipe a Likebook Mimas, I advice to install the 3.0.2 .img from a computer, following the instructions previously presented at this thread, and after, use the 3.1.1 .zip (last known firmware) to make the update on the device itself.
About the wallpapers, I managed to copy the old ones from a 2.x zip and put them on the default wallpaper folder.
In respect to the default reader, I was struggling with the fonts at the 3.0.2 version, and there was some random crashes. At 3.1.1 I found an option to apply in advance my custom typesetting preferences to all .epub files that I would open (click on the gear symbol > More Settings > Global Layout > tick the desired items to keep custom such as font, font size, typesetting and contrast). I achieved better results by putting the "Font Spacing" slider to zero, and by using the default sans-serif chinese font (no need to know chinese, that font renders well western text and suits good to the e-ink screen). Thus, the default reader app satisfies my need for an uniform reading appearance by now.
