When you simmer or slow roast tomato sauce over several hours, the sugars in the tomato release and caramelize which helps to offset acidity. If you’re finding the cooked sauce is still acidic, you can try adding other sweet vegetables such as finely grated carrot, sweet onion, or half of a raw potato (which you’d remove before serving).

That they pack your tomatoes with lemon might mean you need to actively neutralize the extra acids, which you can do with milk or cream, or just a little baking soda as you suggest. Probably not more than a pinch, though, or the sauce could lose its brightness.