I guess I am old school, tri-tip is a Calif thing, originally cooked over an open grill, no marinade just garlic salt fresh ground pepper and dash of garlic powder, then throw on hot grill cook both sides then foil or finish indirect when temp reaches 135º pull and let rest for 5 minutes. I do add a little of one of my rubs and and good dash of my fav steak seasoning, so the mine is heavily seasoned. I foil mine while resting to trap all the juices, use for au jus. Cut the meat on the bias and enjoy.
I have smoked tri-tip in the smoker then finished on the grill, however remember smoking was initially used for cheap cuts of meat to elevate their flavor and tenderness. Tri-tip while not expensive is not a cheap cut lacking flavor or tenderness, smoking isn't necessary. If you want smoke throw a big 3" by 3" chunk of wood on either in your charcoal or gas grill, let it ignite and burn for few minutes then smother the flame and put it back on the heat but in cooler spot it will smoke nicely, if ignites again, smother and set it on a piece of foil to keep from igniting. The charred chunk has to get hot or the smoke will smell like creosote.