The two nights the tours spends in Seville aren't enough. It's a large city with a lot of sights, and they are scattered. Adding more time there (maybe two more nights) would be the first way I'd use part of your extra time, because it would all be productive, with no time spent traveling to a new city. In suggesting that you add two nights rather than three or more, I'm allowing for the fact that the tour will help you see part of the city more efficiently than you could manage it yourself.
Actually, tied for #1 as the best way to use extra time you have available would be to arrive in Barcelona at least two days early, because spending only two nights there is painfully inadequate.
Cordoba is certainly worth a couple of nights (more in my case). That would be my first choice of a new destination for you.
Malaga is also enjoyable. It has some interesting sights, just not as many and important as the ones in the cities I've already mentioned. I would want at least two nights there, though. One night really just means a few hours. The tour moves really fast, and I don't think you're going to want to keep going at that pace during your post-tour travels.
I haven't been to Gibraltar, Nerja or Frigiliana despite having spent 4-1/2 months in Spain since 2016, because I judged I'd enjoy other places more and wanted a lot more time in the cities your tour goes to than the tour itself allows. I took a lot of day-trips, including a visit to Carmona while I was staying in Seville for six nights.
I have been to Tangier and don't recommend it. Morocco is so wonderful, and Tangier just isn't. Plus it takes quite a while to get there. You have to get to the port from whatever city you previously visit in Spain, and you have to be there in plenty of time to catch the ferry, then you have the ferry ride itself. Then you have to repeat the trek in reverse. And when all is said and done, you have only managed to see Tangier. There are flights to much more interesting Moroccan cities (like Fes and Marrakech) from a lot of European cities, so you can hop over to Morocco during a later trip to Europe. You don't have to settle for Tangier just because you happen to be in southern Spain.
Our poster who lives in or near Murcia has indicated, I believe, that the food in that area is very good but the sightseeing charms are a bit subtle by comparison to many other places. I haven't been to Murcia myself, but I can tell you (having traveled through the area) that transportation from Granada up to Valencia is a slog. If traveling by rail you are routed (on the very fast AVE trains) back to Madrid before heading east to Valencia. The bus takes all day and is $$$ unless you figure out how to break the ticket at the right point (which might actually be Murcia).
Valencia I would rank roughly in the same category as Malaga. Nice, attractive, some things to see, but not as distinctive a destination as Seville, Granada, Cordoba, Barcelona or Madrid.
If you go to Valencia, hilly, medieval Cuenca--which is right on the AVE rail line between Madrid and Valencia--is a very nice stopover for a day or so. It's much smaller than Toledo (which you will probably not have gotten your fill of since the tour only spends one night there), and there are a couple of small but worthwhile modern-art museums.
Other places of interest in Andalucia include Ubeda and Baeza, which are Renaissance towns quite different from what you'll see elsewhere. At least part of the journey to those towns from any direction includes a segment of bus travel.
The rest of the post-tour traveling you're contemplating can be done by public transportation. A car might be helpful to get to Valencia more rapidly, but that would depend on where you were the previous night. For example, you might think renting a car for the 6-1/2 hour drive from Malaga to Valencia would save a lot of time, but AVE service via Madrid is faster by as much as 90 minutes.
