Pat the St. Louis–style pork spare ribs dry, remove the thin membrane from the bone side, and square any loose flaps so the racks are evenly shaped.
In a bowl, mix the dry rub: light brown sugar, sweet paprika, kosher salt, black pepper, garlic powder, onion powder, mustard powder, and cayenne pepper until evenly combined.
Coat both sides of the ribs evenly with the rub, pressing to adhere; let sit at room temperature 20–30 minutes while you heat the smoker.
Preheat a smoker to 250°F with clean-burning heat, and add the wood chunks (hickory or apple) to establish steady smoke.
Combine the apple juice and apple cider vinegar in a spray bottle to make a spritz.
Place the ribs bone-side down in the smoker and cook until the rub has set and the surface is deep mahogany, 2.5–3 hours, spritzing the surface lightly every 45 minutes; the bark should not smear when touched.
Wrap each rack tightly in heavy-duty foil, pouring about 0.25 cup of the spritz into each packet, then return to the smoker and cook until tender, 1.5–2 hours; ribs are ready when a probe slides in with little resistance, bones peek 0.25 inch, and internal temperature is 195–203°F.
While the ribs are wrapped, make the sauce: in a small saucepan, whisk together ketchup, distilled white vinegar, molasses, yellow mustard, Worcestershire sauce, and celery seed; bring to a simmer over medium-low and cook, stirring occasionally, until glossy and slightly thickened, 10–12 minutes, then remove from heat.
Unwrap the ribs and return them to the smoker bone-side down; brush a thin layer of the sauce over the ribs and cook until the glaze sets and is tacky, 20–30 minutes.
Rest the ribs 10 minutes, slice between the bones, and serve with additional sauce alongside.