Going to Northern Spain, but very overwhelmed with options and what to do

You're gonna need a car for the Picos. Cangas de Onis will give you access to Covadonga Sanctuary and the beautiful Covadonga Lakes. Blunes, Sotres and Las Arenas are good bases. Potes, a very pleasant little town, is near the Fuente de cable car that takes you high into the Picos. The pilgrimage site Santo Toribio with a piece of the "True Cross" is nearby.

Asturias and Cantabria are littered with charming seaside towns/villages. Pick one or two. Lots of pretty beaches.
There is fantastic cave art in several caves. My favorite is Tito Bustillo.

Don't be overwhelmed, N. Spain is wonderful there is so much to do. Go to the Picos, hang out on the coast. Churches and monasteries abound. Do your research lots of good info out there. These should get you started:
https://www.turismoasturias.es/en/
https://turismodecantabria.com/
brad

Santiago de Compostela, Bilbao and San Sebastian.

Northern Spain covers a large area, from Galicia in the west, to Asturias, Cantabria and Basque Country (coastal all of them), plus northern Castille and León provinces such as Zamora, León, Palencia, Burgos...and then Navarre to the east. Very different one from the other, different architectures, languages, traditions, foods, drinks, climates (well, it does tend to be rainy, mainly on the coast). Asturias is big enough for one+ week, then you may decide to visit Galicia or Basque Country, two very different worlds. Spain is a country of contrasts, of four official languages, of many climates, arquitectures, history...and quite mountainous for a large part of the country, so take your time to choose well in northern Spain. As a Basque, my recommendation would be too obvious...

So people have given you options and this confuses you more. Most of us do research for our trips and have many of the same concerns. The trick is the research, then the hard part is choices. Which is a decision, which is very hard for a lot of people.
After reading your post, I simply googled “best places in northern Spain”. There were many results. You can do this too and then delve into what is available and make your choices. This will take some time and effort. No need to wait on replies on this forum.

Decide how you plan to get around as that will affect where you go. For me a rental car affords the most flexibility. Then select a route from your starting and ending locations. For northern Spain there is a coastal route (A Coruña-San Sebastián) and an inland route (Pamplona-Burgos-León-Lugo) If you start along the coast finish along the interior. Of course there are the Pyrenees where there are fewer roads or direct routes.

Once you have a route, look at the towns along them and see what sparks your interest. There are plenty of small towns and villages, so no need to visit a large city unless you want to. Only you know the places that fit your preferences. Where I might spend a night isn’t necessarily where you’d like to spend a night.