Backlight fix and full guide is available on my Github repo:
https://github.com/merken/Inspiron7586
Also, I totally overlooked one critical thing: I kept booting from the OpenCore entry that was added to my UEFI BIOS, not sure what this thing does, but the solution was to boot from the UEFI internal hard drive...
So basically, I had 3 boot options at a certain point:
- OpenCore
- UEFI internal SSD
- UEFI bootable USB (Catalina installer)
Not sure what the OpenCore entry was for, but it was the reason my changes never stuck around, I believe it is a cache of some sort?
I removed the entry from the BIOS and stuck with the SSD UEFI entry, which boots from the macOS drive's EFI partition (obviously).
The OpenIntelWireless drivers (via Airport or ITLWM) are still glitchy, I use the HoRNDIS workaround for this (plugging in my smartphone and tethering the WIFI)
Anyway, I now have a (more or less) functional HackbookPro, thanks to all who contributed!