I’ve used citric acid in the past. It doesn’t take much to add flavor and I would only use about 6 teaspoons in about 90 ounces of maltodextrin/dextrose/table sugar (I was making bulk batches). I’m not sure what that translates to if just making a single bottle but a small pinch is likely all you need.
If the sweetness is your real issue, maltodextrin is much less sweet than other sugars.