Thank you for the suggestion. However followed the SDDM install instructions (all three steps), rebooted and - exactly the same problem. If I try and log in as the i3 or xfce user it again immediately logs me back out to the SDDM login screen.
Would very much appreciate any more help. As I mentioned in original post, some various logs that might be relevant:
inxi output: http://sprunge.us/CVXoPY
My /var/log/lightdm/lightdm.log can be viewed at: http://sprunge.us/ns5oUV
My /var/log/Xorg.0.log can be viewed at: http://sprunge.us/jjgApS
A ~/.xsession-errors file is created (from the unsuccessful log in I believe), which can be viewed at: http://sprunge.us/6rBGaF
Running coredumpctl also brings up a number of errors with /usr/lib/Xorg, /usr/bin/xfce4-power-manager and /usr/bin/python3.9 as shown at: http://sprunge.us/QeyMCi
Very grateful for any help.