Preheat the pellet grill to 250°F using hickory pellets. Set it up for indirect cooking so the meatloaf can smoke without sitting over direct heat.
In a large bowl, stir together the breadcrumbs and milk. Let them stand for a minute, then mix in the egg, diced onion, Worcestershire sauce, salt, pepper, and garlic powder.
Add the ground beef and 2 tablespoons of the ketchup. Mix gently with your hands just until combined. Avoid packing the mixture tightly, which can make the loaf dense.
Shape the mixture into a loaf on a sheet of foil or a grill-safe tray, leaving the top and sides exposed to the smoke. Place it on the grill and smoke until the center reaches 150°F on an instant-read thermometer.
While the meatloaf smokes, stir together the remaining ketchup, apple cider vinegar, and brown sugar in a small bowl. Brush the glaze over the top and sides of the loaf.
Continue cooking until the center reaches 160°F. Transfer the meatloaf to a cutting board and let it rest for 10 minutes before slicing.