Preheat the smoker to 250°F using pecan, oak, cherry, apple, hickory, or your preferred smoking wood.
Line a rimmed sheet pan or disposable foil pan with foil. Place a small wire rack over the pan and coat the rack lightly with nonstick cooking spray.
Heat the butter or olive oil in a skillet over medium heat.
Add the finely diced onion and cook for 4 to 5 minutes, stirring occasionally, until softened.
Add the minced garlic and cook for approximately 30 seconds, just until fragrant.
Remove the skillet from the heat and let the onion and garlic cool for 5 to 10 minutes.
Combine the panko breadcrumbs and milk in a large mixing bowl. Let the mixture stand for 5 minutes so the breadcrumbs absorb the milk.
Add the eggs, Worcestershire sauce, ketchup, mustard, BBQ rub, black pepper, and cooled onion mixture to the bowl. Stir until evenly combined.
Add the ground beef and ground pork.
Using clean hands, gently mix everything together only until the ingredients are evenly distributed. Do not knead or tightly compress the meat.
Transfer the mixture to the prepared wire rack.
Shape it into an even loaf approximately 10 inches long, 5 inches wide, and 2 1/2 to 3 inches thick. Smooth any large cracks without packing the loaf tightly.
Insert a leave-in thermometer probe into the center of the loaf if using one.
Place the pan and rack in the smoker.
Smoke the meatloaf at 250°F until the center reaches approximately 140°F, usually 1 1/2 to 2 hours.
While the meatloaf smokes, combine the BBQ sauce, ketchup, brown sugar, apple cider vinegar, and Worcestershire sauce in a small bowl.
When the meatloaf reaches approximately 140°F, brush half of the glaze evenly over the top and sides.
Continue smoking for 15 minutes.
Apply another light layer of glaze and continue cooking until the center of the meatloaf reaches 160°F.
Remove the meatloaf from the smoker and let it rest uncovered for 15 minutes.
Transfer the loaf carefully to a cutting board, slice into thick portions, and serve with any remaining glaze or BBQ sauce.