Your timing estimate is correct. The loin will Carryover as the outside meat temp continues to heat the center. If 155°F is the desired result, pull it 5-10°F early and let it coast up as it rests. Fat cap is your choice. I leave it on, some trim or score to get more smoke and rub to the meat. Below is a Topping/sauce my family has used many years...JJ
Apple Pork Topper
3C Apple Sauce
3T Dijon Mustard
3T Brown Sugar
2T Apple cider Vinegar
1tsp Rubbed Sage
1/2tsp Black Pepper
1/2tsp Salt
1/8tsp Cinnamon
Optional: 2-3 Apples, peeled, diced and sauteed until golden brown and tender in 2T Butter.
Place all in a pot and simmer on low until thick but still pourable. Adjust sweet/salt to taste. Spoon over Pork Roast, during last 30-60 minutes of Cook time or when an IT of 110° is reached. Raising the Smoker temperature will caramelize the topping a bit...BUT...Pull and rest the meat, Uncovered, when the IT reaches 135 to 145°F. The hotter surface will cause a 10° Carryover rise and overcook the meat if you go to a higher IT. Serve the remainder, hot, over the sliced pork at the table...JJ