I re-flashed the microSD card and to explore the sddm display manager I used
sudo apt-get install lubuntu-desktop^ sddm
as you and the the instructions on the Ubuntu Raspberry Pi page suggest. (https://ubuntu.com/download/raspberry-pi)
However, when booting the system hangs and so I have to press Alt+F2 to get a terminal and login manually to start X server.
I then get the screenshot below.
The system configuration is very good as once I had added
to usercfg.txt I had video with audio playing out of the box.