Yeah DC suburbs provided the tipping point but they weren't that Republican by vote margin once you factor in Alexandria and Arlington. Richmond suburbs were the clear GOP base.
Alexandria and Arlington both voted for Carter that year, went for Reagan by pluralities in 1980, and flipped to Mondale in 1984. They clearly tilted Democratic by that point, and Republican strength in Northern Virginia was already beginning to slip. You are certainly correct about the Richmond suburbs (Chesterfield and Henrico Counties). They are also the reason why Goldwater was within single digits in Virginia in 1964, as he crushed Johnson in those two counties.