Just a few places you might like to try in Portugal:
Coimbra - beautiful old city. The Uni Library is a must see.
Condeixa-a-Nova - check out the Roman sites.
Batalha & Fatima - religious sites, maybe stay at Ourem.
Sintra - we stayed at Queluz & took the train/tram. Cascais is nice for R&R but has grown, smartened up hugely of late & become more exclusive/expensive.
Palmela & Setubal - wonderful old fortress', now Pousadas. Take the ferry from Setubal, over the Sado & amble down the coast roads.
Sagres & Cabo de Sao Vincente - Portugese Lands End on the SW tip.
Estoi - stay at the wonderful Pestana Pousada.
Tavira - nice Pousada & boat trips.
Then Spain -
Ayamonte is interesting, on the Guardiana; fishing/Sardine canneries historically.
El Rocio - like something out of a Western Movie.
Jerez - needs no intro. We prefer to stay in Cadiz old town.
Arcos de la Frontera - amazing old town on the edge of a cliff.
Ronda - home of bullfighting, with museum. Beatiful site & some great local rides.
Cordoba - must see the old bridge & Mezquita
Granada - the old City & the Alhambra. Triumph sometimes use the A395 road up to the Sierra Nevada for testing.
Lorca - stay at the new Parador, with great views. We quite like the Parador at Mojacar Playa though - often inhabited by VAG/Porsche with test mules. Very relaxing.
& finally to Cartagena - as you say, the Roman sites & circuito.
Sorry, too much for one tour already & that's just scratching the surface, without getting into the red hot centre. There is so much do do/see inland the options are almost endless. Having an old riding buddy living near Denia meant we did Iberia regularly over the last 25 years. Some great riding territory, although the speed camera has emerged of late. You may also be familiar too with the aggressive steep speed bumps that have been laid as you enter some towns/villages, where speed control traffic lights were getting ignored.
Anyway, enjoy the planning & the trip idc ............ KEN