We’ve done that route but not in Summer months.
Yes, plenty of sites in inland Spain but aires are not as plentiful as France. The campsites we found OK but generally not to the high standard you’ll find in France.
As for your timeframe?, how long is a piece of string. Yes, no problem getting to Portugal for a week/10 days and back to Santander in a months window, but as already suggested it’s probably best not to try and squeeze in too much. You’ll do Cherbourg to French/Spanish border at Irun in a couple of longish days and quite easily if you take the toll route, and that leaves you with 3+ weeks for the rest of your plans.
Again, I’m not sure regarding booking of sites in advance, we never had to as we travelled ‘out of season’ so to speak.
In any case you’ll enjoy it, I’m sure you’ll get more advice from members who have more experience than us in high season.