I cook a LOT of flats.
First thing I want to know is what temperature are you cooking at. I cook at 300+ as most of the comp guys who use a UDS do. Don't worry about injections. Completely forget them for now. If you want to add flavor, add some in a broth to your foil.
Season the brisket and put it on the smoker. Once it "looks" cooked (~70C) wrap it TIGHTLY in aluminum foil. Cook it until it is probe tender (~93-99C). Just stick the thermometer through the foil.
Pull it out and rest it for an hour or so to serve. If you want to return it to the smoker to firm up the bark... do it before the rest and refoil it.