So this worked - Once I’d got past the initial setup Nightly asked me to set up rewards again and then had the dropdown available with all countries.
However, I also ran into a different issue- it asked if I wanted to import bookmarks and settings, but there wasn’t a Brave option available, only an ‘Unidentified Chromium Browser’.
I tried to import from that but clicking the button did nothing, so I skipped it. Now the regular ‘Import bookmarks and settings’ doesn’t have an option to import from the regular Brave installation:
I’m less bothered about rewards than moving my setup manually over to the Nightly installation but if you want me to try anything else just for testing let me know.