Another option would be to “turn left” after Villaviciosa and take the route down to Oviedo. This is before Gijón, but might fit into your schedule as well.
If you are more interested in a city celebration, this would be a great place - wonderful restaurants and just a very beautiful city. There’s a real splurge place to stay there, too. The
Hotel Reconquista in an 18th century building. I’ve stayed there as a tourist, and I see that it is now a Eurostars hotel. In my experience, Eurostars does a really great job with nice hotels in old buildings!
If you do walk down to Oviedo, you can easily rejoin the Norte by taking a route (different than the one you came south on) that goes directly from Oviedo to Avilés in a day. I’ve done this several times and the route is now much better marked and makes for a fairly easy day. Or of course you can switch to the Primitivo in Oviedo. So many choices, but no bad decisions. And don’t get me started on the detour to the pre-romanesque church of Valdediós that comes after Villaviciosa. If you’re a fan of ancient architecture this is one not to be missed.