>but VP9 support is absent (or not exposed in many common Linux paths) so YouTube is sloppy [...] but YouTube 4K is generally VP9 or AV1 rather than HEVC
I installed linux yesterday. Youtube doesn't let you backtrack to VP9 in the user profile setting. It serves AV1 by default now for all resolutions. Bummer if you're on older and/or low end hardware.
Maybe some browser extensions can force VP9?