2.5 weeks travel in Spain -- need itinerary help

That itinerary sounds pretty fast to me, even for just "check-list" tourism. So much time lost just to unpacking/repacking, check-in/check-out, traveling, etc.

I find so much of Spanish life/culture/vibes are off the tourist tracks, and for the purposes of relocating I'd want a lot more time to get to know places—I spent nearly two full weeks in Madrid before even putting it on the list for potential future moves. (Though tax differences already made it the only viable option.)

For your (possible relocation) purposes, those times may be adequate. From the tourist standpoint, you're going to be short in Barcelona (only 2 full days with a big chunk of one of them devoted to Montserrat) and Seville (only 2 full days with a day trip to Cadiz).

There's not much rail service into and out of San Sebastian/Donostia and Bilbao. I suspect when you take a look at the schedules, you may decide to choose based on the least-bad departure times. The total time spent on trains doesn't look as if it will be very different, whichever way you go.

To travel between Bilbao an San Sebastian, your fastest option is a bus. The local train takes a convoluted route and makes a lot of stops, but it is rather scenic.

Unlike the typical pattern elsewhere in Europe, Spain's faster trains do sometimes sell out, so I'd recommend checking all the trips you plan to take, with an eye to whether a bunch of trains for the rest of the week are already sold out.

Great info regarding the trains.

It's so hard because I want to see so much so I don't know what to cut out. :) I am the most excited about going up North but also want to see Madrid and Seville. Would you take Valencia out of the equation?

Their Digital Nomads Visa?
I have a penpal who loves between several countries through the year. She seems to be able to case a city as a prospective long term location in 2-3 days. She has standards and walks certain neighborhoods, sees how people dress, rides the local transport and eats local foods and decides from there.
As I have aged, I would be looking at annual weather and how accessible greenspace is. Cadiz is supposedly popular as a small city, with a dry environment, but it seems to be desert and often has wind and flooding issues. What about Jerez?

Valencia is the current hot spot for expats. Whether you see that as a positive or negative is personal perspective.

Yes! The Digital Nomad Visa. Yah I was thinking Cadiz just as an interesting day trip, not to live there but open to any other day trips from Seville or I could stop for a night or two in Cordoba down from Madrid.

Scudder -- good to know about Valencia. Oh it's such a puzzle!! :)

I do like to just walk around, eat and take in a just a few sites, get a feel for a place. Not wall to wall activity. Rather roam around so I know I don't have enough time for all of it. I loved Barcelona, but eager to see other parts. You should see what I started with..lol.. I cut out Malaga, Granada, because it's just not feasible.

Well, Malaga and Valencia are reasonable considerations for relocation, but if the north calls, then start there and give it a bit more time.
Once in Spain, explore further.
I don't know if you have been pricing accommodation lately, but I know in the main cities we are mentioning in Andalucia (where there is a lot of promotion of this new visa) apartment and Airbnb month to month pricing is considerably more expensive than pre and during pandemic. Perhaps do a quick accommodation search of some of your cities and see if any can be crossed off the list for those reasons.
You may want to consider the bus from Valencia to Barcelona. It stops at the Barcelona Airport before the city. Lovely scenery, 3 hours. Hotels nearby.
To be honest, your outlier is Seville.
If your flights aren't booked, consider flying in to Barcelona, and home from Madrid. Move Valencia to the beginning and cluster with Barcelona, and then you can cluster the north with Madrid.
I can appreciate you want to do it all, but you need to determine which is the focus....tourism or relocation searches. Spreading yourself too thin is a disservice to either.

Haha yes, I am definitely trying to do/see too much and I need to simplify and focus -- kid in the candy store who needs a month. Really good suggestions though, thank you.