I have a nice test image that goes up to 10000 nits. It's a series of squares with given brightness, going from 100 to 10000 nits.
https://alexfry.github.io/ACES_ODT_Candidates_Examples/imagesDiagnostic/PQ_100_to_10000nits.avif
With Plasma 6.3 or earlier it was displayed with gray levels rising up to 465 nits, which is max what my monitor can show, and all the other rectangles were just pure white, as expected.