Combine salt, garlic salt, celery salt, black pepper, onion powder, paprika, dill, sage, and rosemary in a bowl.

Lightly moisten roast with water using a damp towel, then pat with prepared rub. Refrigerate for the flavors to fully come together, 2 hours to overnight.

Preheat an outdoor grill for high heat and lightly oil the grate.

Sear roast on the preheated grill until brown on all sides, then transfer to a platter. Reset the grill for indirect medium-low heat (if using charcoal, move coals to the outside edges of the grill pit).

Return roast to the grill; cook, turning occasionally, until desired degree of doneness reached, about 1 1/2 hours for medium-well. Transfer to a cutting board; cover with aluminum foil. Rest before carving against the grain into thin slices, 10 minutes.