$7 a lb for brisket, OUCH. I haven't bought one in awhile, but then again not surprising.
I think trying to rush a brisket with a higher smoke temp and ending up with at least the flat overdone or tough is wholly unsurprising. I doubt it was the rest time, it's just that the fat within the meat didn't get enough time to render well. You're in a gray zone between smoking and grilling.
Personally with any large piece of meat, I've gone to resting in a cooler for at least a couple hours wrapped in towels. I just set my schedule back far enough to allow for this.
However if for some reason I was pushed to not do this, I'd at least wrap in foil and let the meat rest for at least 30 minutes before I go to slice or pull/shred it for serving.
Trimming a whole packer brisket -- frankly I've been caught far more times on the opposite end, of trimming not nearly enough. Usually when I get to that point I'm too excited to get the meat on the smoker. However I've learned from making that mistake too often, it's better to trim that sucker down to have (guessing) no more than 1/4" of fat cap -- or even less. Most of the fat you really want for flavor is already inside the meat.