I also like your idea of keeping things simple. The hard part, as always, are the edge cases. If the tax is associated directly with sugar in some manner than diet sodas won't be taxed. Another possible albeit silly argument is why not tax sugars in fruit?
Note: Im a software engineer. Not a dietician. I really don't know a detailed difference between artificial sugars and natural sugars or how our bodies process them.
Running with this idea, there'll eventually be a federal conversion table where any given chemical would be translated to their mass in sugar. From there, tax calculation's simply:
sum((sugar_conversion_lookup[ingredient] * mass) for ingredient in product) * tax_per_gram_of_sugar
Because total sugars and added sugars are not the same thing. It makes more sense to tax added sugars because they are clearly worse, and it strains Medicare/Medicaid budgets.
Adding grape juice or apple juice to a snack or drink would still be considered an added sugar on the nutrition label under FDA guidelines[1].
The reason they do that is often to make sure "sugar" isn't the first ingredient in the ingredient list, which have to be listed in decreasing order of weight[2].
Note that the ingredient list and the nutrition facts label are two different things.
1. https://www.fda.gov/food/new-nutrition-facts-label/added-sug...
2. https://www.nia.nih.gov/health/how-read-food-and-beverage-la...