Your sirloin roast is going to be good!!!
A lot depends on how you like your beef...
Rare, med rare, med and so on...
Are you using a thermometer to check the roast temperature? Or the finger press method?
I'd start with a basic rub, salt, pepper, garlic and onion to begin with. This will let the beef be the star of the plate. I have never been one to inject meat. Brining, on the other hand, I have been known to do, just not with beef.
From your explanation, it sounds as if you went too far(temperature) on your previous beef smokes. But I am just guessing at this point. Knowing which cuts you have tried would give more information to determine where you need to pull the roast out of the smoker.
I smoke my beef roasts @ 225-235 on my WSM, over a combination of hickory and almond/pecan. The timing of the smoke is something I don't go by. I go by the internal temperature to know when it's done.
The finish temperature depends a lot on the cut. Sirloin is a leaner cut, so as not to dry it out, you need to remove it from the smoker at a lower temperature. I personally like my beef on the rare side and I would pull the sirloin at around 120. Let it rest for 30-45 minutes and place it over a high, direct heat on my kettle. This is what is known as a reverse sear. This will give the roast a nice, caramelized finish.
Medium Rare: 130-135
Medium Well: 145-155
As to mopping or saucing, I would wait until the last 30-45 minutes of the smoke. This will allow any sauce to glaze and thicken on the meat.
I'm sure others will come into this thread with their methods and ideas.