Itinerary advise

You have 13 nights if I am counting right. That's not enough to travel all the way from Porto to San Sebastián and back. In that timeframe, I would definitely forget about the Basque Country.

You could, just about, do a loop that includes NW Spain and reaches all the way to Madrid, like this (counting nights):
- Porto 3
- Douro 1
- Salamanca 1
- Madrid 3
- León 1
- Santiago de Compostela 3 (including time to explore Galician coast)
- Porto 1

Skipping Madrid this time would make things easier for you.

Funny how Bilbao is practically always misspelled as Bilboa...and I still do not know why!! :) The Basque Country is way far east to make a logical itinerary, distances are long and spending two nights in Donostia-San Sebastian and one in BilbAO would not make much sense. Instead I´d rather spend three nights in Bilbao with side trips to D-SS (can be seen in one day) and to Rioja or the coastal towns in between, in case you decide to keep your visit to the Basque Country.

When in Madrid, do day trips to Segovia and Toledo.
You don't need more than two nights in Porto to see that city, take the train up the Douro River and stay the night a one of the villages there. The train tracks run along the north side of the river.

For Porto, stay at The InPatio Guesthouse, it is the BEST B&B ever.

Not sure how you’re traveling, so am not certain about public transportation to Barreiros. Very near it is As Catedrais beach. It’s best seen at low tide to get the best views of the rocks. Many roads in that area are narrow and not necessarily in the best shape. The road along the coast running by all the beaches isn’t too bad. You have a couple cities on your list, San Sebastián and Bilbao, I’d save for another trip. Since you will be in Galicia, Lugo and its UNESCO World Heritage site Roman Wall around the town is worth stopping at. Since you plan on going to Salamanca, both Zamora and León are very much worth visiting. Lots of potential places to see and there’s seldom enough time to see them all.