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.