Bilbao/Santillana del Mar/Oviedo/ Ribidero/ Santiago de Compostella.
(Then due south through Porto, Coimbra, Lisbon to the Algarve.)
As regards the northern Spain part of the trip, we LOVED Santiago, and then on the homeward leg back towards Bilbao, Salamanca was also amazing….
If I can be any help, let me know.
Good luck with the plans and have a great trip.
DC