It's starting to sound like your 'nomodeset' fix works because you've still got a modesetting GPU enabled (the Intel GPU). As such, I expect that's the only GPU that Wayland will find by default. But if your monitor is plugged into the Nvidia card then Wayland is displaying to an unused card/connector. So as far as GDM is concerned it has booted on a machine with an Intel GPU and no monitor connected, hence the black screen. That's my theory anyway.
Thanks again.
It's starting to sound like your 'nomodeset' fix works because you've still got a modesetting GPU enabled (the Intel GPU). As such, I expect that's the only GPU that Wayland will find by default. But if your monitor is plugged into the Nvidia card then Wayland is displaying to an unused card/connector. So as far as GDM is concerned it has booted on a machine with an Intel GPU and no monitor connected, hence the black screen. That's my theory anyway.