Spain and Portugal in 15 days

Possibly Coimbra or Tomar would work for you.

Everyone's travel style is different, but I am not a fan of two night stays, because of the amount of time it takes to check in; check out and move to the next location. With that in mind, I would consider staying four nights in Sevilla and then do Cordoba as a day trip from there. The conventional rule of thumb is: 2 nights = 1 full day; 3 nights = 2 full days etc. Your itinerary has a lot of running from place to place, rather than exploring and enjoying a location.

I'm with you to a certain extent, but we've had numerous people tell us that Córdoba is worth more than just a day trip and experiencing it after all the day-trippers are gone is magical. So, I'm inclined to stay there longer. Our original itinerary cut out Granada all together, adding 2 more nights in Sevilla, but everyone kept saying we can't miss Alhambra. It's tough to listen to everyone's advice while also making our own decisions. Definitely not married to our itinerary. Still exploring options.

I do agree with your friends about spending a night or two in Cordoba. It is magical at night, and there are other wonderful sights to see in addition to the Mezquita. I personally would not omit Granada and the Alhambra, either. I know, it sounds like everyone is telling you what to do based on their interests. But, Seville, Cordoba, and Granada are considered the most interesting Andalucian cities regarding historical and cultural sights. They are also very different from each other architecturally and visually.

I also think you should add a night or two to Barcelona. Don't forget you will be getting over jetlag when you arrive in Barcelona. And Barcelona is a large city with many interesting sights, and many of those sights require advance reservations. So you may not be able to see a lot in one day, depending on what time slots are available. Although maybe December won't be so busy? Are there Christmas markets in Barcelona? That would make it busy.

Ultimately it is your decision based solely on your preferences. Most of the time you’ll get it correct. On most trips we find we would have liked to stay at one place longer while wishing we hadn’t spent as much time in others,

Last month we spent three nights in Córdoba and it was our fourth time visiting the city. One difference was its patio festival was going on and visiting them took most of one day. The city really is worth more than a day trip.

While we didn’t visit them there are a few interesting towns outside of Porto. https://portoalities.com/en/beautiful-villages-near-porto/

Coimbra is a lovely university town that’s an easy connection point from Porto. We spent 2-3 nights there which was a nice amount of time to stay. You didn’t mention if you’re interested in wine but many people go to the Douro valley from Porto. It maybe a quieter time since you’ll be there in early December but perhaps a nice time to visit.

As others have suggested you can also add a night to Barcelona and Sevilla and just visit Porto if you’re not finding something to your suiting.

If you can change your return flight and fly back from Lisbon you could consider southern Portugal which will be easy to get to from Sevilla.

I loved Evora in Portugal but I don’t think it fits very well into your current plan.

My personal choice would be Coimbra for 2 nights, with Figueira da Foz as a day trip if the weather looks decent.
Coimbra feels like the safer and richer December choice: smaller than Lisbon or Porto, very atmospheric, walkable, historic, and still lively enough in the off-season. It gives you a slower Portugal stop without feeling too quiet or empty.
Figueira da Foz is lovely if you want the Atlantic, long beach walks and a very calm coastal mood, but in December it can be windy, wet and sleepy. I would hesitate to make it the main 2 - 3 night base unless the ocean is the main thing you want.

So my vote would be:
Sevilla → fly to Lisbon
Lisbon → train to Coimbra, 2 nights
Day trip to Figueira da Foz if the weather is good
Coimbra → train to Porto

That way you get the best of both: historic inland Portugal and a taste of the winter Atlantic, without making the trip too dependent on coastal weather.

Agree with the suggestions to add a night to Barcelona and Seville. Failing that, perhaps consider renting a car on arrival to Lisbon and driving to Porto via a quick stop to walk the walls in Obidos, then Nazare’, Tomar and/or Coimbra. If choosing between Tomar and Coimbra — a vote here for Tomar!

Have a wonderful trip.

More time in Seville
Ignoring Madrid and Toledo

Madrid and Lisbon are both beautiful cities. I would cut a night from Porto and the other Portugal city to get time in Lisbon, not sure you can fit Madrid in, but it is a beautiful city with excellent food and the museums there are world-class - the Prado, the Thyssen & Reina Sofia (la Guernica is there).