Here's Belly's famous Dr Pepper marinade. Several of us use it. Try to let it work for at least 24 hours before cooking.

I sub worchestershire for soy and lemon for lime sometimes.

Sounds like you only got part of the whole brisket. Probably has very little fat on it. You should either get some beef fat from the market or else drape the meat you have with bacon to keep it from drying out. Also use a spray of 50/50 oil and either Dr Pepper or lemon juice every hour or so to keep the meat from drying out too much. Next time look for a whole packer trimmed brisket. Weight should run from 7 to 15 lbs. With a good fat cap. It will do better with the long cooking times for low and slow.