So in short: best use stock HW and SW setup - only adjust Kodi settings as outlined eg here https://wiki.libreelec.tv/configuration/4k-hdr
Just a comment/observation about the recommended Whitelist given in that Wiki.
I am using a 2GB Pi 4B and was having a problem with a 720p HEVC @ 25fps file that had sound but no picture - at first I managed to get it working by disabling DRM Prime but couldn't fathom out why a simple 720p HEVC file wouldn't play when I have no problem playing 2160p HEVC at all refresh rates.
After a lot of head scratching, and at one point about to put this question to the forum, I had a light bulb moment.
My Whitelist contained the resolutions/refresh rates listed in the Wiki but I noticed 1920x1080 @ 25 is missing from the list.
I tried adding this to the Whitelist and, low and behold, the file now plays even with DRM Prime enabled.
I am aware that there is a discussion about allowing Kodi to double a 25fps file to 50fps, or something relating to this, but I am taking the approach if it works then it works.