Do you actually need help from interior designer to choose upgrade options? My wife just chose the material/style (for flooring, countertop, backsplash, for example) that looks good to her. And yes, builder designers won't be helpful in that sense. Al they can do is maybe give some suggestions on color matching.
I guess for my wife, it wasn't too complicated. She wants modern/contemporary style and mostly white. Fortunately, all the upgrades turned out pretty much exactly as she imagined them to be, so she was very happy with the final product.