The flickering thing is because you’re using Wayland and Nvidia GPU. If you switched to X11 (losing HDR support, unfortunately), the flickering goes away. However! I would recommend updating your system to KDE 6.1, which I believe has the explicit sync fix.
KDE 6.1 just hit Arch repos like 3 days ago. You’re likely on 6.0.5. Switching between X11 and Wayland is effortless just change the default manager in SDDM settings.