North Spain

Not far from San Sebastian is Guernika, site of an infamous aerial bombardment by Franco and his German allies in the civil war.
Moving memorial museum. Worth a stop of couple of hours.

Driving westward near the coast:
Bilbao for the Guggenheim
Gijon, pleasant port town with maritime heritage and historic fisherman's quarter
Oviedo home of Fernando Alonso (F1) and his museum of F1 cars.
A Coruna, port city with medieval quarter.

Dropping down further south:
Burgos - cathedral where El Cid is buried.
Leon - history and beautiful architecture.

Do you have a specific reason to visit Irún? It's not the most charming place. I would prefer Hondarribia next door. Might be more expensive, though.

The answer would depend on how much time you’re allotting to drive from point A to B and whether you plan on driving along the north coast or taking an inland route. Going to San Sebastián from Toledo and Cuenca is really out of the way if you want to end up in Santiago de Compostela.

Then there’s the question of cities or small towns and villages. If the mobility issue also pertains to longer, level walks, then smaller towns might be better since oftentimes parking can be found near the main plaza. Before finalizing an itinerary check out the proximity of parking lots and garages in the places you intend to visit to the sites you want to visit.

For example, in Hondarrbia the historic center is up on a hill and the parking lots are nowhere near it. While there is an elevator to avoid the walk up the hill, there’s a decent walk from the parking lot to the elevator. A taxi is also an option.

Toledo is VERY hilly. Even if you take a taxi to the town center, there are still a lot of up and down. Parking not near the town center. Hondarribia is much smaller and although somewhat hilly, inside the town is fairly flat. We were there in April and found parking near the town walls. This is a pretty drive from San Sebastian to Hondarribia along the coast.

Our favorites in Northern Spain (so far):

Chillida Leku
Hondaribbia (pinxtos @ Bar Gran Sol)
Cudillero (lunch)
Bilbao (Guggenheim)

And, nearby, Laguardia. Have been twice and want to return!

There are so many lovely coastal towns in Asturias and eastern Galicia. I'd particularly recommend Ribasedella, which ice flat and has a lovely beach. Also Luarca, Llanes, and the small city of Oviedo. A base near Arriondas or Cangas de Onis will afford you a chance to peek into the Picos de Europa without much strenuous activity.

Cudillero means a steep descent from parking into the town. It's very pretty but also has become a tourist magnet in recent years.

There are very many rural inns that would make lovely stopovers along that drive. One we liked very much on our most recent trip is this one, not far from Luarca, but it's one of many:

https://www.hotelrural3cabos.com

ON an earlier trip across the north coast of Spain, from Santiago to Santander, we stayed here and I highly recommend both the hotel and its restaurant, in far eastern Galicia.

https://www.hotelego.es

I think you know that Cuenca is very hilly; we set two nights at the Parador last November, but one would have sufficed. Visiting the town, across the bridge over the gorge, entails quite a bit of steep walking. IF time is limited, I would certainly eliminate Cuenca for more time in Asturias or Galicia..or Cantabria or the Pais Vasco.

When we were in Cudillero last September we parked in a pay lot right on the harbor. It was a short walk into the town but it was flat. We had a delicious lunch at Tienda La Gijonesa and bought an interesting piece of local pottery at a tiny shop around the corner. It was a lovely afternoon.

Thanks to everyone who replied. Sorted itinerary now. Toledo, Cuenca, Donastia San Sebastián, Abuli then Santiago de Compostelo and probably somewhere on way home. Appreciate all the helpful suggestions