Nice resource, our cycle holidays are also a mix of highs and lows with some interesting planning decisions, I've also done that road with the nasty climbs round Laredo. Give me a nice mountain pass any day! We've met packs of dogs in Spain and Portugal a couple of times, not funny.
I would vote for not bothering to take a tent to Spain outside the main holiday months. A good portion of the campsites will probably be closed ( I've come across closed campsites in a June and September). Of course wild camping may be an option. In the days of more favourable exchange rates we used to mix up camping with Paradors/Pousadas and also little 3* hotels like the author.
Personally I've always found Spain quite a challenge with the often empty landscape and scorching temperatures (although I did manage to get snowed on in the lovely Picos once in June). The rio de plata route sounds interesting. I've also cycled up through Portugal: Lisbon to Santander) and that was really very special.
Cycle-touring, Expeditions, Adventures, Major cycle routes NOT LeJoG (see other special board)
Helpful reading. The Picos sound as if they can get pretty cold even in summer and since i am crossing them from Santander to join up with the via de Plata further south, I think I'll take a lightweight tent and more than a summer weight sleeping bag.