I will only speak to San Sebastián. The joy for a first-time traveler is probably visiting the bars of the old quarter, going in and out to try a different pinxto in various places. You might have great weather, but think about how it would be for you if it is raining..ducking in and out of crowded bars with an umbrella...
I'm sure Porto would be fine, and Bilbao okay,..but a river tour in the rain, with little visibility?
I don't' think I would go again at that time of year, but you might be okay....could you choose a different itinerary. maybe focusing on the south, or even including some larger cities where there are more indoor activities? (I don't know where else on the Peninsula you have been before....)
This is probably not relevant for you, but we took a nice trip at the same time of year in 2024; flew to Madrid for about 3 nights, then train to Cuenca (three nights but would advise two you have no car); train to Valencia for 9 nights; rented car and drove to Denia for a 5-night stay.
There are so many places within an easy drive of Madrid--not only the famous Toledo, el Escorial, and Segovia--but others easily reachable with a car....and that's a good time to be in Madrid.
A real outlier would be one of the Canary Islands....I know that's probably not on your radar but Lanzarote deserves to be more well known to American tourists... (at least, I loved it!!). and it's a good time to go there; not yet high season.