Set belly skin side down. Using a knife, score the belly flesh in a checkerboard pattern 1/3" deep so roast will cook evenly, and you can get additional flavors deep into the meat.
Flip belly skin side up. Using a paring knife or Jaccard, poke dozens of holes through the skin all over belly. Poke until you tire. You want as many tiny holes as possible to help render the fat under the skin. Using the jagged edge of a meat mallet, pound skin all over for 3 minutes to tenderize, which will help make skin crispy when roasted.
Turn belly and Smear minced garlic all over the surface of the meat, then generously season with all-purpose seasoning. Next, arrange herb sprigs down the middle of the belly.
Roll belly around herbs; tie crosswise with kitchen twine at 1/2" intervals. Trim twine. Transfer roast to a wire rack set in a rimmed baking sheet. Refrigerate roast, uncovered, for 1-2 days to allow the skin to air-dry; pat occasionally with paper towels.
Let Porchetta sit at room temperature for 2 hours or while the smoker comes up to temperature. Preheat smoker to 275°F. Season Porchetta skin with kosher or sea salt.
Place seasoned Porchetta on a rack in or over a baking sheet and put it into the smoker, rotating the pan and turning Porchetta occasionally, until an instant-read thermometer inserted into the center of meat registers 175-185°F. If the skin is not yet crisp, remove the Porchetta from the smoker and roast it in an oven heated to 500°F for 10 minutes more. Let rest for 30 minutes. Using a serrated knife, slice into 1/2" rounds. Remove the sprigs before eating.