I get the impression that your definition of ‘too hot’ probably excludes anywhere south of the Massif Central, let alone Spain in the summer. We’re on the southern edge of it, in the Lot Valley and I think you’d find here uncomfortably warm forseveral months of the year. On the other hand we can leave home just before 10 am and have lunch in Spain or be on the Med. Maybe the Pyrenees would work - cooler at altitude but closer to Spain.
We love both countries, but where we live is wonderful and, so like many of our international neighbours, we only visit Spain in Jan or Feb.