Hello Matthew. When a sauerkraut soup turns out too sour, you can easily balance the flavor without starting over.
First, you can add a small pinch of sugar or honey to counteract the sharp acidity. Another effective method is adding dairy, such as a splash of heavy cream or a dollop of sour cream, which coats the palate and mellows out the sour notes. Alternatively, adding a peeled raw potato can help absorb some of the excess acid as it simmers. I hope this helps improve your weeknight meal.