Tried, it didn’t change anything.
Update: I’ve installed Nvidia beta driver 495.29.05, gone through the guide again just to double-check everything’s the same, then rebooted. I have almost the exact same issue as described in my first post in this thread, to which I’m replying now. The only difference is that the cannot open display: line is repeated several times in ~/.local/share/sddm/wayland-session.log now, instead of just once.
If anyone has any insight into this cannot open display: problem, please help me. I’ve been at this for months with no progress.