I've been to Italica, though not on camino. I'd say it's worth waiting for depending on what the flow-on affect would be of losing a day, but I'm a former tour guide in Rome, so I like Roman sites more than most. Italica has a lot of mosaics in situ, which is the highlight of it. There's also a small amphitheatre, and it's the birthplace of emperors Trajan and (probably) Hadrian.
Plus, you'd get to spend an extra day in Seville, which is my favourite large Spanish city. If you haven't been before, don't miss the Real Alcazar (a kind of poor man's Alhambra), the Cathedral (the largest Gothic church ever built, if I'm not mistaken, with a lovely orange tree courtyard), and just walking around the historical neighbourhoods, soaking up the atmosphere and enjoying the magical light.
¡Buen camino!