Indeed, I am another user with the infamous optimus, reverse what-have-you setup 
My challenge is that I need to run both cards in parallel so the Intel GPU powers external monitor 1, and nVidia external monitor 2.
After some troubleshooting and a bit of reading up, it looks likethat this setup requires some sort of “reverse” protocol which isn’t implemented on Wayland yet.
Good thing is that the recent fixes in Plasma 6.0.2 fixed an issue where the system would freeze when coming out of powersafe which means my X11 setup works as stable as pre Plasma 6x.
So all is good for me with X11.