Ah, so KWin uses Intel but plasmashell tries to use the NVidia GPU. The question is only why, it shouldn’t do that. But the NVidia driver failing to provide buffers usable on the Intel GPU is definitely a NVidia driver bug… I think you can report that on their forum.
Setting it to NVIDIA actually works! Well, kind of. Plasma starts as it should, and my external monitor works as well. However, I can no longer use my internal laptop display
You don’t have to make KWin only use one GPU, you can just list them, separate with colons, like this:
KWIN_DRM_DEVICES="/dev/dri/card0:/dev/dri/card1`
The first one will be used for compositing, the ones after that are just used for controlling the displays attached to them