OK
I am getting convinced that even if the Wacom Drivers are not totally responsible they have something to do with it.
Yesterday there was an update for Wacom. After that the issue changed habit.
I had to reboot 3 times to get the wallpaper to appear. If you look at the wallpaper option in system preferences there is a spinning wheel for a very long time before anything appears.
Anyway, eventually the wallpapers kicked in, but this was coincidental to the Wacom Center seemingly hanging and then popping in.
Today, I booted up and two grey screens on iMac and Cintiq. I rebooted and got the same. The Wacom Centre was freezing, but eventually popped in. I quit and still two grey screens. Frustrated I switched off my Cintiq and low and behold the wallpaper for the iMac popped on screen. It was in Dark mode which I never selected and when I reselected Dynamic it changed to my usual preference.
I am not sure if this is playing a part, but the Wacom device has issues based around this "Installer Progress" screen.