That link was great. I stumbled across it yesterday when trying to fix HDR on my AVR X4200. The step which worked for me was:
"Disable the video conversion in the AVR."
Something as simple as that (default) setting was killing HDR, resulting in washed-out colors.
Now to see if my new RPi 4 with LE can reliably play 4K content for entire movies (my S912 with CoreElec struggled).
Cheers, Carl