Preheat oven to 350°F. Place a non-stick 9 x 5-inch loaf pan in the sink and spray well with cooking spray. Prepare a baking sheet by lining it with aluminum foil. Set both aside.
In a small bowl, stir together 1 tablespoon of the yellow mustard with 1/4 cup ketchup and all of the brown sugar.
In a large mixing bowl, break up the wagyu and the ground pork using your hands and mixing as you go. Add the chopped onion, Kosher salt, black pepper, garlic powder, remaining yellow mustard, remaining ketchup, eggs, cubed bread, and milk. Again, use your hands and mix very well.
Loosely pack the meat mixture into the prepared loaf pan, pressing down only once you've filled the pan. Pour the prepared glaze over the meatloaf making sure to reach all sides and all corners.
Place the loaf pan on the foil-lined baking sheet in case of spill over. Bake until the loaf registers 165°F with an instant read thermometer, about 1 hour to 1 1/2 hours depending on your oven.
Remove the insert very carefully using oven mitts as metal will be very hot. Allow the loaf to drain before gently sliding it onto a serving platter. Allow the meatloaf to rest 5 minutes before slicing.