Preheat the pellet grill to 250°F. Line a rimmed baking sheet with foil for carrying the meatballs to and from the grill.

In a large bowl, combine the ground beef, breadcrumbs, egg, milk, diced onion, Worcestershire sauce, garlic powder, salt, and black pepper. Mix gently until everything is evenly combined. Avoid packing the mixture tightly.

Shape the mixture into 16 meatballs, about 1 1/2 inches wide. Arrange them on the prepared baking sheet with a little space between each one.

Place the meatballs on the pellet grill and smoke until their centers reach 160°F, checking with an instant-read thermometer. Transfer them carefully to a plate.

Pour half of the marinara into a saucepan and warm it over medium-low heat. Add the smoked meatballs and turn them gently to coat. Keep the remaining marinara for the rolls.

Heat the broiler. Split the hoagie rolls and arrange them cut side up on a baking sheet. Spoon the reserved marinara over the inside of each roll, then add four meatballs to each one. Top each sub with two slices of provolone.

Broil the subs until the provolone is melted and lightly browned in spots, watching closely so the rolls do not burn. Serve warm.