San Sebastian to Santiago De Compostela

Thinking back on my trip in that area in 2017, i’m not certain that is really possible to do in a day. When we left Santiago we went over to Muxia for a couple hours, then up to Cathedral beach for a couple hours, and spent the night in Ribadeo. The next day we went to Cangas de Onis and Picos de Europa for about 4 hours and spent that night in Santillana Del Mar. The following day we were in Hondarrbia. Where you will lose time is once you get off the autovias. That roads in the north are very rural and you’re not going to be driving much more than 30 miles an hour on them. Most are barely 2 lanes wide. You’ll also lose time once off the autovia gong into A Coruna. Again, local roads, traffic, and looking for parking.

I know you're speedy travelers, but I hate to see just 3 nights in Madeira. It is really gorgeous, with varied scenery, lovely vegetation and great walking opportunities along the irrigation channels (levadas). And you have to fly to get there--though I realize you'd otherwise be flying straight from Lisbon to Barcelona.

So, let's see...it's a long long drive via the coast and your 7.5 hour driving time estimate is optimistic; 8.5 hours is more realistic - even the highway is surprisingly winding, not all of it comes with the usual 120 kph speed limit. And forget about taking the old non-highway road with such time pressure, it is unbelievably curvy. My stomach still remembers a first, early 2000s trip in Asturias with my parents.
I would break it:
- if only one night, around Cudillero, a pretty harbor (no need to detour to Oviedo if you won't visit)
- if two nights, then Santillana and Ribadeo make good 1-night breaks

To me, there is no need to visit La Coruña if short on time. It is a fine place, but in my opinion, no match to, say, San Sebastian from a sightseeing standpoint.

Now, where to "steal" one or two nights... I'd look at your Portugal itinerary
- I'm no expert of that Portuguese region, but the Coimbra-Tomar-Nazaré section of trip looks very generous in comparison, I don't think Tomar deserves as much time as Santiago for instance
- Going Nazaré-Evora-Lagos-Lisbon is more efficient that sleeping in Lisbon twice

In short: I would not consider driving from San Sebastian to Santiago in a day with sightseeing in mind.

We did this itinerary in reverse. Dropped off Portugal rental car in Porto. Taxi across the border to Vigo, Spain. We figured this all out using AutoEurope's website. We were quoted a 1000E drop fee for pick up Lisbon, return in Barcelona. Our hotel in Porto found us a taxi driver, non English speaking so cost less, no problems, , to take us from Porto hotel to Vigo care rental agency instead of taking the train. This was easy to do. Drivers I had researched online quoted much higher fees. The drive was mostly on highways.
We stayed in Baionad and then up to Santiago de Compostela, then stayed in Leon, Santillana del Mar, then San Sebastian.
Then across north side(France) of Pyrenees, staying in two locations, then Collioure, France on Mediterranean, then Begur, Spain in Costa Brava, finally turned car in at BCN, and stayed a week in Barcelona. This was, however, a six week trip.

Way too many places.

You can do Evora on a day trip from Lisbon.
We did Santiago de Compostela in one day on a day trip from Porto.

Thank you for suggestions. Working to tweak itinerary to include longer stays in main cities with more day. trips. Will post updated itinerary for comments when complete!