I was having the same no signal black screen issue since Plasma 5 and finally fixed it by simply setting “nvidia_drm.fbdev=1” and “initcall_blacklist=simpledrm_platform_driver_init” alongside “nvidia_drm.modeset=1”. After doing this, it no longer gave the black screen and loads the desktop when selecting a Wayland session.
I don’t see that you or anyone has mentioned these, so maybe give that a try and see if it helps.
I also have a 120hz monitor with a couple of side monitors, one at 100hz and one at 60hz, and this fixed the issue for me, so there’s that similarity between our setups.