I understand that it’s an absolute brightness standard, not like the relative levels in SDR
The standard is also relative brightness actually, though displays (luckily) don’t implement it that way.
why does it end up washing out colors unless I amplify them in kwin? Is just the brightness absolute in nits, but not the color?
It depends. You might
have a driver bug. Right now only AMD has correct color space communication with the display, that doesn’t work correctly on Intel and NVidia yet
have a display that does a terrible job at mapping the rec.2020 color space to the display
be just used to the oversaturated colors you get with the display in SDR mode
Why does my screen block the brightness control in HDR mode but not contrast?
Because displays are stupid, don’t assume there’s always a logical reason behind what display manufacturers do. Mine only blocks the brightness setting through DDC/CI, but not through the monitor OSD…
Why is my average emission capped at 270nits, that seems ridiculously low even for normal SDR screens as comparison
OLED simply gets very hot when you make it bright over the whole area, the display technology is inherently limited when it comes to high brightness on big displays