After reading this thread I took a look at the settings options on my
DIY home-built iMac 27" 5K monitor - which has an iMac Pro 27" screen driven by an R1811 driver board sold by the Chinese specifically to work with an iMac or LG Ultrafine 5K screen.
This drives the iMac Pro screen exactly as one would hope for, producing a gorgeous display for my Mac mini M1.
As expected, enabling HDR in Display Settings on the Mac produces the washed out screen.
But buried in the R1811's internal settings menu (under Advanced) is an HDR - Off/Auto/HDR toggle (for HDR10).
Which I've just set to Auto and tested with YouTube, and it works amazingly!!
The Mac HDR button has to be unchecked (setting it on produces a doubled/garish effect), so the monitor driver board is handling the HDR detection.
One of the Chinese specs of the R1811 I had read says the HDR10 setting only works with 4K, not 5K, and the colour accuracy is probably somewhat inaccurate, but all the R1811's menu settings all still function and I can definitely get a good-enough picture to watch the YT movies.
And all the desktop colour settings for normal windows/wallpaper etc look as usual.
Very impressive.