I found my issue. It was the OSID to XSID and _OSI to XOSI renames. Once disabled, I had audio but lost brightness overlay (hotkeys still worked though). Was trying to work out how to keep both but no luck thus far.
@RehabMan, could you please take a look at my issue?
I'm still trying to get audio working with another config. Not sure where the issue lies as it's independent of DSDT and kexts used.
The config I'm using now has working audio but, the brightness overlay doesn't come up or move the slider on Displays prefpane plus it freezes the system if pressed too quickly in succession.
The other config which remedies this, doesn't allow audio though. I can restore audio by booting into a linux live USB but it won't survive a cold boot, only a restart.
I'd prefer to get the brightness overlay and slider link working plus restore audio. I don't mind if brightness resets to max every reboot.