Hi, here's some feedback. If you've got 28-35 days, that's a solid amount of time that I think you should be able to see at least a couple places in Portugal as well. I'd say reserve 5-7 days for that. A couple suggestions on the Spain itinerary.
I don't think you need 3 days in Bilbao, I'd trim that to 2. It's only 1-1 1/2 hours from San Sebastian, and you could even do as a day trip if you wanted. Good amount of time on Barcelona, and if you want to do any day trips from there, you could go to Girona, Montserrat or the Dali Museum in Figueres. I would skip Cartagena, especially given that it isn't going to be beach weather. If skipping Cartagena, I'd then say take the train Valencia to Madrid (1 1/2 hours) and train from there on to Sevilla (2 1/2 hours) as it will save you some travel time. I would definitely include Sevilla in there. While in Madrid, I strongly recommend a day trip to Toledo, the former capital, which is just one-half hour by train. Don't cut out Granada - you should definitely see the Alhambra, and make sure to book far in advance as it has limited space. Slice a day or two off of Bilbao, and potentially a day off of Valencia as well.
Enjoy!!