So, for the easy way you can just install Magisk.
It will need a copy of boot.img which you can extract from an update.
As there seems to be a problem with fastboot boot, you can save it in recovery (/dev/block/mmcblk0p8) and boot recovery.
Your mods will have to be as Magisk modules (so that you can do Treble updates).
Code:
E60P54_android:/ $ su
E60P54_android:/ #