After pasteurization, a settle period from 1 to 6 months will help round out the acidity I have found. You could ameliorate by adding some sugar to balance the acid. Adding some water will bring the sake out of genshu strength to regular strength, but like mead, it would require tasting small measured amounts for tasting and then scale up.
Sometimes before pressing and pasteurizing, adding more steamed rice for a few weeks sweetens up the sake because the enzymes are still there but the yeast has retired. Depends on how much rice is added.