Preheat the pellet smoker to 275°F. Set out a rimmed sheet pan that will fit inside the smoker.
In a large bowl, combine the venison, ground pork, egg, breadcrumbs, onion, milk, thyme, salt, and black pepper. Mix gently with your hands just until combined. Shape the mixture into a compact loaf and place it in the center of the sheet pan.
In a small bowl, stir together 2 tablespoons of the maple syrup and 2 tablespoons of the Dijon mustard. Brush half of this mixture over the meatloaf, reserving the rest for later.
In a bowl, toss the carrots, parsnips, and potatoes with the olive oil, the remaining 2 tablespoons of maple syrup, and the remaining 1 tablespoon of Dijon mustard. Arrange the vegetables around the meatloaf in a single layer.
Place the pan in the smoker. Cook until the meatloaf reaches an internal temperature of 160°F in the center and the vegetables are tender, about 1 1/2 to 2 hours. Check the loaf with an instant-read thermometer rather than relying only on the cooking time.
Brush the meatloaf with the reserved maple-mustard mixture during the last 15 minutes of cooking. Remove the pan from the smoker and let the loaf rest for 10 minutes before slicing. Serve with the roasted vegetables.