how to remove acid in a dish
Simmering/cooking the tomatoes longer will help. Remove the portions of the soup that you think would suffer from low-and-slow cooking, and simmer the broth/tomatoes (I would put them in a crockpot on "low") for 4 hours. Taste. If necessary, bump up to six hours.
This will avoid having to add sugar.
You can also add a couple of tablespoons of grated carrot to the broth before you simmer.