I've tested and reproduced this error on several systems using maxwell and pascal cards on the latest install image.
Everything runs fine, but upon rebooting into the fresh OS, the first boot dialog and login manager will fail to start and hang on the openrc checklist
the workaround is to either set iGPU to primary in bios, and install with modesetting/intel.
once you have a working desktop, create a new BE, and back up your current xorg.conf to something like:
% cp /etc/X11/xorg.conf /etc/X11/prev.xorg.conf
Activate the new BE and reboot. install nvidia drivers and utilities:
% sudo pkg install nvidia-driver nvidia-xconfig nvidia-settings
run nvidia-xconfig, reboot into bios, switch your monitor to the nvidia gpu, and change pcie to primary adapter. Boot into lumina.
At this juncture lumina may have lost all of your panels or be similarly screwed up. Not to worry: just right click, hit Preferences > All Desktop Settings > User Settings > General Options and click restore lumina and desktop defaults. Reboot and everything should be hunky-dory.
Hope this helps