To add to the options, you can fly from Santiago (or Paris?) to Seville or Malaga. So it would be possible to start south and work north.
I think the key question is where do you need to be at the end to fly home? If it's a Spanish airport such as Madrid then you might want to end there. If it's via another British/European country, it might not matter as you'll need a connecting flight anyway and these are possible from most parts of Spain.
If all options are on the table you could go from Paris to Andalucia, then fly to Galicia, then fly to Barcelona and finally a train back to Madrid. But I'm just making that up since I don't know where, if anywhere, is already fixed.
Valencia is wonderful, but not - in my view - necessarily a priority for a first visit. Alacant is fine, but after the castle not a great deal to see (even Murcia province is better!). Since you mentioned it first, I'm assuming SantiagoDeC is a must for you. If not, it would make an easier routing to exclude it this time.