Just this weekend I installed Manjaro after having tons and tons of issues with SUSE since the Plasma 6 upgrade. I have a laptop with AMD integrated graphics, which Plasma is running on, so your issues may not apply to me. But if I run any apps on the Nvidia GPU in full screen, I do get the flickering issue.
My biggest issue I have is that no matter what distro I use, as soon as I install the proprietary Nvidia driver, my system fails to boot like 20% of the time. It just freezes during boot with no error messages or warnings that I can find. But once it boots, it works.
I’m not sure how to run Plasma on my dedicated GPU so I can see if I have the same issues you have. But with my current setup, it works.