Whole wheat panko is interchangeable with regular panko, right?
Interchangeable, yes. But you are wasting money using panko in meatballs. Panko is specially made to result in extra-crisp coating of fried and oven-fried foods. You're paying extra for that property. For the panade in meatballs and meatloaf, where bread crumbs are soaked, use up whatever stale bread or crackers or unsweetened cold cereal you have on hand.