Yeah I would suggest sticking with Trixy if you want a stable and reliable experience, I’ve been running Debian for well over 2 years now as a daily driver, I have not had the need to switch away from the stable branch.
Regardless have you tried reinstalling the KDE Packages?
sudo apt purge --auto-remove '*plasma*' '*kde*' 'sddm'; sudo apt install kde-full kde-standard kde-plasma-desktop task-kde-desktop sddm; sudo reboot
^The above set of commands will likely reset your KDE settings, wallpapers, etc so bear that in mind before running this if you need to back any settings up.^