Weather-wise, Normandy and Brittany are pretty similar to the Basque Country in mid-summer. I tend to prefer interior towns rather than coastal towns; the latter can be a bit too touristy for my taste. Therefore, I'm not the best one to suggest specific spots. Caen is a transportation hub, but it was obliterated during the war, and I don't think it would suit your taste.
Another possibility would be Galicia, in NW Spain. It, too, tends to be coolish and overcast by comparison to points south. I liked A Coruna, which seems to trend sunnier and warmer than the rest of Galicia. The major destination in that area is Santiago de Compostela, of course, but it is pilgrim-packed and not on the coast. It is also one of the least-sunny places you can find. Pontevedra is quieter, sunnier, and on the coast.
Everyone, including me, really likes Gdansk. The historic district was beautifully rebuilt after the war. However, it has been discovered for sure. It has two outstanding museums and many smaller ones, but it feels more "city" than I think you want, even aside from all the tourists.
I bet there are some interesting options in northern Germany, but I don't know that area.
As you can see, I'm having trouble setting aside my preference for not-hot areas for a mid-summer trip.