Have you tried visiting the north of Spain? I'm from Galicia, in the northwest. Fantastic food, culture, monuments, nature, and no overtourism except perhaps in Santiago de Compostela in the peak season. The same can probably be said about Asturias, Cantabria or León. Most foreign tourists come to Spain attracted by the Mediterranean beaches, sunny climate, and stuff like flamenco or bullfighting, which are southern or eastern Spanish things. The north and northwest are highly underrated (of course I might be biased, but pretty much everyone that comes here from abroad tells me that as well).

Come fast, before the high-speed rail lines connecting Madrid to several cities in the north and northwest are finished in 3-4 years from now. I cannot guarantee the situation will hold after that, my intuition is that bad infrastructure is a major reason for the relatively little attention these areas attract.