Brazilians, Americans, Australians, Mexicans are solely (immigrant) nations and therefore solely nationalities, not ethnicities.
Wrong.
You are right though, that these nationalities may become ethnicities in the - may I say far - future
It's already happening.
Just because most (?) Brazilians don't identify with a Brazilian ethnicity doesn't mean that there aren't those that do.
All that said, I don't think ethnicity is a useful concept other than to identify political and historical currents
That makes it an extremely useful concept.