I second the advice to avoid both.
It's not that they aren't "nice" cities but I think you will be left with the thought: "why did I bother?". Oviedo, is really not a tourist city although it does have its fair share of cultural sites, great eateries and watering holes. However, if the weather is hot, it becomes cloying and if it's raining, it rains most of the day. Not so Gijon; generally cooler and being coastal the weather changes more throughout the day. As with (almost) any city, in order to make the most of it, you have to study the "Secret City" beforehand to get the most out of it. Usually this means tagging along with a local because in both of these cities, it's the bars and restaurants that are less obvious that offer the best quality.
If you do decide to go to Gijon, though, if you stick to Cima De Villa in the middle of the coastal city, you won't go far wrong for entertainment, good food and atmosphere.
Cudillero: is fine for a stop, as are Luarca, Navia and Ribadeo. However, booking on the fly is not recommended to avoid disappointment. It may already be too late. In Cudillero you will have to leave your bike on the sea wall but it will be safe there. I'd avoid weekends.
Luarca: Stay well away on or around the 22nd. Local festivities (San Timoteo) cause complete chaos and not only will you struggle for accommodation but you will struggle full stop, for everything.
If peace and quiet is what you seek then check out
https://www.hotelrural3cabos.com/en/
I'd avoid the A8 altogether and use the N632/634. If you go on the DC you will miss the best part of that coast, especially if the weather is good.
Along the way I can recommend the following stops in order of appearance:
Oviñana, Cabo Vidio.
https://www.turismocudillero.com/faros/faro-de-cabo-vidio/
My great-grandfather built the lighthouse. He was an arse (allegedly) and he only built the main building, not the horrible 60's addition to the east of the lamp. Eateries are of a very high quality, Bar Rest El Café in particular, El Regueirin does a menu of the day (go hungry) and there should be a new brasserie opening called El Tizón by then. Or just chill in Cai Milio. if you see a GS there with Sp number plates, cameras front and back, a TT (instead of a NAV), usually parked inside the terrace, then ask for Fred but if you roll up on UK plates, I'll come and say hello anyway.
Cabo Busto Lighthouse.
https://www.turismoasturias.es/descubre/naturaleza/rutas/senderismo/cabo-busto
If you like cakes there is a famous cake shop there but there will be a queue.
Puerto De Vega.
https://www.google.com/search?q=puerto+de+vega+spain&client=firefox-b-d&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiEqLHZ9-DiAhXB8eAKHf02Al0Q_AUIECgB&biw=1404&bih=686
Fabulous quiet little fishing village for a coffee/beer or just chill and leg stretch. You can park right on the sea wall.
Viavelez.
https://www.google.com/search?q=viavelez+asturias&client=firefox-b-d&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj_vvrT-ODiAhVHxYUKHZWADdIQ_AUIECgB&biw=1404&bih=686
Same again but smaller.
Tapia de Casariego
https://www.google.com/search?q=tapia+de+casariego&client=firefox-b-d&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjVs4L6-ODiAhWs4YUKHasxDJAQ_AUIESgC&biw=1404&bih=686
Bigger than the previous two. I'd recommend going right down to the port and ramp area where you can park your bike easily enough.
And lastly. We found a really cool place to have a swim (dangerous beach/riptides, be careful) called Praia Do Castros just after Ribadeo but just before Rinlo (another great little village to visit with a coastal walk).
On the clifftop ( a mere 10m high) where you park your bikes on the side of the road there is a temporary "chiringuito" run by two brothers-in-law. One's Argentinian who will serve you up an ice cold beer, a plate of ribs and a plate of freshly cooked octopus unsder a parasol that will have you begging for more.
I'm sure Arsey will provide some photographic evidence because I have recently deleted 3,000 photos from my cloud accidentally including the ones from last years trip
If the weather is nice, this really is a great stop...... as before, weekends will be crowded but in August it may be academic.
https://www.google.com/search?client=firefox-b-d&biw=1404&bih=686&tbm=isch&sa=1&ei=423_XN3iF874aNitgIgN&q=praia+dos+castros&oq=praia+dos+castros&gs_l=img.12...15781.18640..20635...0.0..0.77.147.2......0....1..gws-wiz-img.gsm8l6bPNkI
Enjoy.