Two weeks too late, but a new Plasma 6 ISO is up again today.
Still uses Calamares Qt5 based as the installer.
Moved to a new sound theme, ocean-sound-theme will replace oxygen-sounds in Plasma 6, so already added now.
kf6, Plasma 6 and the KDE Apps are all build from a git checkout on Sept 17.
ISO sets the wrong wallpaper, doesn't use the Midna theme, just uses the Breeze wallpaper, investigating what changed upstream. Workaround used sets the Midna wallpaper again.
Logout/reboot/shutdown from the GUI does not work, results in a frozen system, use cli for now, like Fix found, check the SHA256SUM to get the correct, updated ISO.
sudo reboot, already known for a few weeks, no solution found yet to fix.