Gary,
Thanks for that, it all look brilliant and very informative. There's nothing I can obviously think of that would be causing the icons to not display.
I have had weird issues when using some other programs and then using Prusa Slicer and Cura, e.g. I used Blender 2.79 on the pi to do basic modelling, if then leave the design open and try either of the slicers it doesn't always work. But by the sounds of it you'll have tried running it with nothing else in the background.
What resolution are you running 1080? or 4k? I don't have a 4k monitor so can't test.
Did you backup your profile and delete the folder and contents of /home/pi/.config/cura?
You could also try invoking the appimage with a set 'settings' folder using
../some/path/to/my.AppImage --appimage-portable-config
Other than that I'm out of ideas. Sorry.