Cordoba??

Just my opinion, but I don’t think you need to extend the trip. I feel you could cut a night from each; Granada, Toledo, and Valencia. You could use that time for Córdoba that really only requires 2 nights. Of course, everything is dependent on what you plan to do and see in each place and whether you were going to take any day trips from those bases.

I think Cordoba would be a very good addition. In general, I always think more days will be better. In the time you have between now and your trip, you will read about a lot of places you don't have in your current plans and wish you had more time. This I can guarantee. For example, ten days is very, very little time to see Barcelona and move across northern Spain to Porto. You'll be passing through or bypassing such interesting places as Zaragoza, Burgos, Oviedo, Leon, and Santiago de Compostela as well as many worthwhile smaller towns. I had no trouble filling 89 days in Spain (no Portugal) in 2016 without going to Andalucía or Valencia. Have to go back next year to hit those areas. Hope it doesn't take another 89 days.

So: Bring on more days! And Cordoba is one of Spain's top destinations.

Edited to add: Dontcha love it? On average, Jaime and I think your trip is Just Perfect!

Adding my definitely YES for including Cordoba on your trip. It was our favorite town of our Spain trip, which included many of the cities you mention. It was unlike the others. We very much enjoyed wandering through the narrow winding roads, seeing the Mezquita (again the favorite of the trip), the Roman Bridge, Alcazar and other sights. We stayed 2N and wish we had another night to spare, although most will say 2N is plenty. Have fun - your trip sounds wonderful!

Another vote for adding Córdoba! Personally, I like Córdoba better than Granada and Sevilla. It will be the least crowded of the three, especially if you spend the night, that way you can escape the hordes of people day-tripping in from Granada and Sevilla, who will leave in the afternoon.

While I'm generally in favor of longer trips, you're is plenty long to cover the area. Cordoba is indeed magical and well worth a visit, if only to experience the Mezquita. I'd take a couple nights from Valencia, which didn't impress me as a stop that would justify lingering very much, and use that time for Cordoba.

I've never been to Valencia so am not in a position to have much of an opinion, but I don't think a one-night stop in a city the size of Valencia is worth much. It takes more time to get settled into a large city like that, and it may take time to get from the train station to ones hotel. Plus a direct train from Madrid to Barcelona takes less than 3 hours, vs. about 5 hours (and higher cost) if you travel by way of Valencia.

Thank you for all your wonderful comments and suggestions. We are going to take Valencia out of the equation and add Cordoba! One more question, from Sevilla, should we drive to Granada, then to Cordoba and onwards to Toledo or from Sevilla to Cordoba, to Granada, to Toledo etc. There's probably no right answer, but if someone has driven this, we'd appreciate the benefit of your experience.

I don't know, but I'd ask ViaMichelin.com. Just plug in the various legs for each routing. Your actual driving time will probably be longer, but the comparison should be reasonably valid. Use the "+" symbol to add the midway points.

The Mezquita is the single most amazing thing we have seen in Spain and we have climbed around in Sagrada Familia back when it was pretty much open for exploration and we have toured the Alhambra. Both of those are also amazing, but the Mezquita like Sagrada Familia is singular.
https://janettravels.wordpress.com/2013/09/27/the-mezquita-at-least-the-catholics-had-the-good-sense-not-to-destroy-it-when-they-desecrated-it/

Sevilla-Córdoba-Granada-Toledo is a good order. Sevilla to Córdoba is 75 miles. Cordoba to Granada is 130 miles. Granada to Toledo is 188 miles. It’s all interstate highways in exceptional condition. On the trip up to Toledo you’ll exit the E5 onto the CM42 (CM stands for Castille and La Mancha). Close to that exit in the direction of Toledo is Consuegra that has a nice castle and windmills. On the same road, but heading east is Campo de Criptana and its windmills. Both are worth visiting. The CM42 is not an interstate, rather a 2 lane road. I’ve driven it multiple times and it is in good condition.

Edited: depending on which road you take from Córdoba to Granada, it could be all interstate. The E5 to to E902 would be all interstate. However the E5 to N432 won’t be. The N432 (N=national) will be 2 lane, but in good repair. All the N roads are the old main routes before Spain had interstates. They are in good repair, but will pass through towns. If you have the time, that route would be more scenic than taking the interstate, but will take longer.