My grandma's cookies require 6 tbsp of baking powder how much sugar to use so cookies don't taste butter and salty?

Without seeing the entire recipe (for the ingredient ratios) it's almost impossible to offer a credible suggestion.

Adding sugar alters the chemistry of the dough and will eventually affect the bake and end result.

Six tablespoons of baking powder is a massive quantity for a home cookie recipe, I would be more inclined to cut back the quantity rather than alter the ratios of other ingredients.

One course of action might be to make several batches. Adjust one variable at a time otherwise you won't know how any change affects the final product.

Best of luck.