I do not think your issues are in any way unique. I am also using Plasma 6 on Arch with a 3080 Ti. Initially, it simply booted to a black screen with nothing but a cursor, and no ability to do anything.
Adding the changes to /etc/mkinitcpio.conf per @markus 's reply as well as adding those kernel params to my /boot/refind_linux.conf, and now it boots into a desktop, but I am seeing large amounts of flickering, windows going black, or visual elements flickering in and out. Looking at my installed packages, I already had both xorg-xwayland and wayland-protocols (per @Varen 's reply) installed.
It seems like there are still more steps before what I would consider a usable result. Personally, it was not “obvious” to me when Plasma 6 rolled out that I needed to do any of this.