Tomatoes are acidic, baking soda is alkaline, sugar is neutral. Adding baking soda to canned tomatoes will neutralize the acid but won’t change the taste too much (depending on the amount). Adding sugar won’t change the acidity of tomatoes but will greatly change the taste.

From my personal experience, adding sugar to a simmering tomato sauce for long enough that sugar caramelizes, will make the sauce way better.

Sugar - it doesn’t neutralise but mask the sourness, so the resulting taste is a bit more interesting. Bicarbonate will truly neutralise it but the result is a boring sauce.

I use both depending on the mood, but I noticed that baking soda neutralises the taste of tomato whereas sugar neutralises the acidic taste.

Coco powder is an option I use as well. A little bit will change the flavor in a nice way. (this is often the secret to real Mexican food)

buy fresh tomatoes?

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.

Tomatoes are already acidic, consider non-lemony tomatoes next time if possible.

As for this batch, both sugar and baking soda works somewhat and you can to both.
A little at a time so you don’t change the taste too much.
Onions are acidic too, but much less so than lemontomatoes, adding a bunch of em to the sauce can help.
If you don’t plan on freezing or canning and you’re just making sauce to put on pasta or something, add cream to it to make a sauce rosée, it’ll mellow the perception of acidity a lot too.
Or use in it a chili where the beans, while not chemically buffering the acidity like baking soda would, can help absorb some of the taste.

Heat and time on the stove.

Sugar for sure. While it can push the edge of sweetness too far if you go heavy, adding a bit at a time will allow you to counter the acid without also changing the rest of the flavor your way bicarb can. Sweetness can be easier to “fix” as well. Up your herbs and spices and the sweetness fades, unlike bicarb which still has that distinct flavor under everything else.

Sugar is neutral in PH, but can be used to help balance out overly acidic food. I would be worried that in this case it would just bolster the flavor of the lemon if it is strong enough that you can detect it in the sauce.

Baking soda should work fine and just turns the acid into CO2 and salt, but if it has a flavor of lemon, it sounds like you are dealing with lemon oils in the sauce.

You could try cutting it with some fat (milk, cream or butter), and I would try adding more onion and/or garlic to complement/mask the lemon.

You should probably consider buying a different base tomato sauce in the future.

I’ve always added a carrot.