Pork butt is easy to cook, and easy to keep and reheat, IMO.
I would cook it, rest it, and pull it. Then chill it, reheat it, sauce it and serve it.
Seriously, serving it one day after the smoke is not a big deal, IMO. Pork butt has plenty of fat, so you can reheat it in the oven a day after you smoke it, and it will still be moist and tender.
I would wait until you are ready to serve to add sauce. IMO, saucing it before it is ready to serve could make it mushy.
Other brethren may have other opinions, but mine is pull it after it cooks and rests, and reheat and sauce it when you are ready to serve it.