Solution Found:-
→ Settings
→ Connected Devices
→ Connection Preferences
→ Android Auto
→ Version - hit this 10 times to enable ‘Developer settings’
Select the 3-dots-menu @ top right & click ‘Developer Settings’
Scroll down & enable ‘Unknown sources’
Go back to Android Auto settings & select Versions
You should see - under ‘Version and permission info’ - Roon ARC: 1.0.33
Go back to Android Auto settings
→ Customise Launcher
Enable ‘Roon ARC’
And now it should work.
Not sure why I had to solve this myself, rather than an App / Software Developer…
…and am also not sure why it’s necessary to enable ‘Developer Settings’ to make it work.