If you would get this working, your application's windows would pop up on the login screen, accessible to anyone who walks past - do you really want this?
And when someone logs in, I think any windows started on the login screen will be forced to terminate, so your application would have to detect the user login and somehow re-establish the connection to ...
Okay so I got there in the end!
For anyone out there what I did was completely uninstall anything related to KDE (sddm window manager etc.).
Then it turns out the icons being weird was to do with running X11. For some reason the icon wasn't scaling properly, so it was tiny. I've now switched to Wayland, for other reasons (per monitor scaling mainly), and ...
This is explained by the entry for ForwardX11Trusted in Debian's man for ssh_config, with Ubuntu behaving the same as Debian, but not Manjaro:
If this option is set to yes, (the Debian-specific default),
remote X11 clients will have full access to the original X11 display.
If this option is set to no (the upstream default), remote X11