The Marques de Riscal is awesome to stay at, especially if you’re a fan of architecture. And of course it’s a good base to explore wine in the region. Pintxos-hopping in nearby Logroño is also worth doing.
i wouldn’t skip San Sebastián for it, but would reduce time in Bilbao (maybe just do the Guggenheim). I also suggest stopping at San Juan de Gaztelugatxe, which is along the coast between Bilbao and San Sebastián. I’ve never been to Biarritz or Bordeaux, but those areas in Spain are phenomenal for sightseeing, food, wine ...