Go for it, I think you and your wife will have a very nice time.....I'm definitely leaning towards an overnight trip to SD.
Take an early bus out of Sosua and you should be checked in your hotel in SD by very early afternoon. Walk El Conde, the Colonial Zone, the Malecon.
Adrian Tropical is a nice restaurant for dinner and is located right on the Caribe Sea. Take a horse drawn carriage ride along the Malecon, negotiate the price. Whatever they say walk away and the price will drop.
Jaragua Hotel has nightly entertainment, just some local talent but enjoyable. Play the slots for awhile. Shoot some pool with the wife in the sportsman bar. A "good" night sleep and a leisurely pace to catch the bus back to Sosua.
Whatever you do, don't try to make it a day trip.