Midna-dark is set to be removed, no-one is willing to maintain (it was user maintained), and had no commits for years, so not compatible with current Plasma, thus certainly not part of this release cycle testing.
Whenever changing colors, make sure that both color scheme & kvantum matches the selected color, one light, other dark, will never work.
For Calamares being full screen, but not resizing content, how to replicate? That screenshot, cala looks awful, and should never be that that way. Please also test with normal scaling. And what graphics card does this system use? nvidia? Is it capable of really running wayland, or needs an X session?
For sound, what card? Output of systemctl --global status wireplumber
& systemctl --global status pipewire
?