Since no one rarely seems to have the answer in this forum I will answer myself.
Follow this thread
https://www.mobileread.com/forums/sh...highlight=ROOT and install the towelrootv1 as attached (searched this file for ages on google, it does not exist even if you search for v1 on multiple sites) and this version is indeed thankfully v1 which is the only one that will root onyx ereaders.
After install ereader will reboot
Then install a apk from app store like normal that has the fullscreen functionality like full!screen.
Done. 100% of screen working for reading
I only installed the towelroot, not supersu or any other files so far. Don't know if I have to, atleast the reader is working like normal.