Regardless if defuser was on or off, cooking at 325-350 is fine. I cooked my H/F brisket at 375-380 and it came out great once probe tender. I find, since I'm still a noob, that an IT gauge comes in hand to assist when to begin probing. That is all one should use an IT temp gauge for. One thing must be engrained for briskets, PROBE TENDER.
Begin with fat cap down, and end with it up (if you wrap). Keep trying. Good luck