How much is your car rental rate? I found it cheaper to hire a car with driver in April than rent a car (through taxinvinales). If you are an attentive driver, I don't think driving in Cuba seems to be that tricky other than for poorly-marked directions, passing slow vehicles and watching for bad road surfaces (lots).
Road isn't too bad but be aware the highway stops west of Camagüey and it is single lane otherwise. Top speed is pretty much 80-90 KM/h so your average speeds may be a little optimistic.
As far as Santiago, I found it interesting but not sure if the city itself is worth the hassle. Went out to see El Moro/Morro, Piedra de something to the east, and headed to Baracoa via Guantanamo before turning back to Gibara (where we left the driver) and flew home from Holguin. Maybe save eastern Cuba for another time. I would use the time to spend in Vinales instead.