6 degrees would probably be OK. I'd head south toward Valencia along the coast. Part of our route last summer took us from Valencia through Barcelona. It is almost 9 C in Valencia today The roads were good and I don't remember any issues with wind. I'm not sure how riding it in the winter would be. Our coastline in Oregon tends to be the warmest part of the state. Is it the same along the Mediterranean? Are all the hotels, etc. operating along the coast in the winter? Can you take bus or train back if you don't make a loop?

Good luck on your venture.