I should have noticed this thread earlier, but I did almost exactly the same thing yesterday. A flat at almost 5 pounds from my local BJs Wholesale. In my case, I had it in the smoker (MES) from a little after 7 AM till 5:45 in the evening. IT was 195, but it never got to where a probe went in like warm butter. It was still a bit more resistance than that. Let it rest for half an hour and then ate a mess of brisket last night. Right now my wife is frying up some garlic and onions in oil to reheat it. We'll have smoked brisket fajitas.
Rub was very simple: non-iodized salt, pepper, onion powder and garlic powder. Rubbed it Saturday night and let it sit in the fridge overnight. Took it out to warm up a bit at 6:30, around the time I started the smoker heating.
Bob