If you log in (preferably as another user) to another tty (Cntrl + Alt + F2/3/4/5), and then run dbus-run-session startplasma-wayland, does that work?

Sometimes just startplasma-wayland works, which used to work for me but didn't work for me this time.

Then, you should be able to switch back to the original KDE session, which continues to work normally, at least it does for me. Both sessions should have shared access to the GPU, as well. You can then probably run sunshine in that second instance of KDE, while the monitor can be attached to tty1 or tty2, whichever one is the default where KDE is put.

Then, you would probably want to configure sunshine to work even when the screen is locked, that way you can lock the second session with Win + L, but it can still be interacted with remotely via sunshine.

EDIT: I only am somewhat confident that this setup will work, I haven't tested it personally.

iirc for GPU acceleration you need to make the GPU pretend to use one of it’s ports (you don’t actually need to plug anything into it)

Just use wolf: https://games-on-whales.gitbook.io/wolf/stable/en/user/quickstart.md. Everything is ready out of the box. However, for a KDE session, you'll have to rebuild KDE in the container instead of wolf-ui.

Second comment:

https://docs.lizardbyte.dev/projects/sunshine/v0.21.0/about/guides/linux/headless_ssh.html

There is also this guide, but it is specific for Xorg and Nvidia gpus. But I suspect that if you replace the startx here with startplasma-wayland or dbus-run-session startplasma-wayland, then it will work. It would still be nvidia gpu specific though.