Spain Itinerary, Madrid, Euskadi, Galicia, Andalusia, Barcelona

Three nights in Seville is quite short. That may end up being just two days by the time you get there from Portugal. I also think it's a shame to travel all the way to Galicia for just two nights.

Few people on this forum would agree that a day-trip to Tangier is a good use of your time, whatever Rick may say. You may not be aware that there are flights to interesting Moroccan cities like Fes and Marrakech from a number of European cities. A trip to Southern Spain is not your only opportunity to visit Morocco. It's a fascinating country; Tangier is not one of its best destinations.

This is a long trip; you won't be able to move at a consistently rapid pace for six weeks and may not be in great shape to tackle the sight-rich city of Barcelona in just four nights.

I'm not sure what visiting Gibraltar by public transportation entails.

Bilbao (2?) and San Sebastián(3). Then head west by bus to Santillana del Mar (2 nights with Altamira Cave visit),

There´s much more to do and see in Bilbao than in Donostia-San Sebastian, I´d change the number of nights. And two nights in Santillana is too much, the town can be visited in one hour and you do not visit the Altamira Cave, but a replica that is more a museum than a cave. I´d allow one night in Santillana...or just a day visit. The north of Spain is very rainy (well, not that much this year!) so be prepared for all kind of weather!

@acraven. Thanks. If I eliminate the 2n in Tarifa, Spending only 1 n in Santillana del Mar adds one more night elsewhere. I could move those nights, increasing Santiago de Compestela from 2n to 3n, Sevilla from 3n to 4n and Barcelona from 4n to 5n. I agree better Moroccan sites are elsewhere and that will be another trip. Good points.

@Mikel. Thanks. I can plan those 5 nights for Bilbao-3n and San Sebastián-2n. A college teacher 40 years ago really liked Santillana del Mar, so I have wanted to go there and didn’t make it there back then on a previous trip. We did visit the Pont-d’Arc replica cave in the Ardeche in 2016 after it opened. It was still interesting. The Picos wouldn’t be very scenic on a rainy day so may need to be flexible or just deal with the rain. Maybe the Altamira Caves could be skipped and I add a day to Galicia.

Any insights on buses from Arcos to Ronda? Or driving from Santillana/Potes to Santiago de Compestela?

Driving from Bilbao to Santiago is, by far, the most convenient way to visit that area. The roads are excellent, traffic is light, parking is easy: there is nothing to worry about really - except the high cost of rental cars these days. Also, an IDP is needed in Spain.

Otherwise, a day visit is enough for Santillana, in my opinion.

If you really want to see caves, I´d ignore the Santillana Altamira museum and would, no doubt, visit El Castillo and Las Monedas caves, very close to Santillana and they are the very real thing, with real cave paintings over 34000 years old. Or any of the other caves in the area. https://cuevas.culturadecantabria.com/en/

Thanks for the "Euskadi" name for the Basque Country on the title of your post, first time I see it on Rick Steves!!

Another vote for visiting a real cave. Be there and experience the actual art!

Not a bad plan, but I have a few suggestions.
1)Forget about Tangier, believe me it is not worth the cost and effort and waste of time. Morocco is great, but Tangier is not. Plan on a separate trip to Morocco visiting Fes, Marrakesh, Rabat and more.
2) 3 nights in Seville is not enough. We did Santiago de Compestela on a day trip from Porto, Portugal and that is all you need there.
3) 4 nights in Granada is probably too much, cut to 2 days.
4)You didn't mention visiting Segovia, just north of Madrid. Don't miss it there is a great castle and an intact Roman Aqueduct that was still functioning until the second half of the 20th Century.

Mikel sent a great link to real caves in the Santillana area so will likely visit those. Thanks. We have visited caves in the south of France. Replicas at Lascaux and Pont d’ Arc were well done and still amazing. The real caves have the authentic musty smell.
@geovagriffith- We are no longer considering Tangiers. We will likely get in somewhat late to Santiago de Compestela and may hike one day, or take a day trip hence 3 nights. But will consider other options in Galicia. I have been looking at renting a car to tour Las Alpujarras so will post separately about possible overnights there - Pampaneira, Trevelex? A day trip with a rental car eliminates the hotel/AirBNB transition, although it would be worth spending a night or two in the Sierra Nevada.
Segovia is very nice. I was there 40 years ago about the time I was selling cuckoo clocks in Titisee.

I think the deal with Santillana del Mar is that it is an architecturally very picturesque medieval village that now has a lot of tourist shops. I was glad to visit it briefly in 2016, but I'm sure the experience 40 years ago would have been much, much nicer.

As for Santiago de Compostela, I disagree that it's only worth one night, especially on a trip of this length. You're going to appreciate having some days when you have time to just mosey around. Santiago has a large historic district.

Other places in Galicia that I liked: A Coruna, Betanzos, Pontevedra, Tui (near Portuguese border), Cambados and Combarra. They vary a lot in size. At the time of my visit (2016), Combarra seemed the most obviously foreign-touristy; it's also the smallest. I thought the city of Vigo had little to recommend it, so that's a place I'd skip.

I agree with Mikel on going to cuevas Las Monedas and El Castillo instead of the simulation at Altamira. Both are an easy walk apart and you can do both in a single morning or afternoon. I'd also recommend Cueva Tito Bustillo in Ribadasella (I haven't been yet, but will visit next month). Its art is said to rival that in Altamira. But, access is limited and it might be hard to get a reservation.