[SOLVED]
The update went through smoothly and fairly unspectacularly.
Had to change the theme, weather widget 2 is broken and icon-only taskmanager scales the icons extemely tiny when using small spacing.
The Nvidia driver under Wayland is better than before, but sadly still flickers like hell in Steam-client and games, so I still stuck with X11…
Edit:
I have just stumbled across a problem…
Dolphin Custom actions error: not authorized to execute
Update:
Thanks to @Olli it’s solved.
It seems that the .desktop files must be executeable now!
I have just checked it in my backup. It was definitely not necessary before.