The grade of the meat matters, to the extent that a Choice, or Prime is easier to get tender. But, the same rules apply. You have to apply the right amount of heat, over the right amount of time, to get it tender. I think you are still over-complicating things here.
First off, don't rub the night before, there is no need, and potentially, some problems. Rub it about an hour before you start cooking. For me, this system has worked over and over with Smart & Final Select packers, usually from Excel Packing.
1. remove from cryo, wash and trim of fat.
2. go fire up cooker
3. rub the packer
4. wait for fire to get stable, this takes about an hour, it rarely takes less
5. plop the meat into the cooker, close and let it run
6. check after an hour, see where cooker is, at grate temperatures between 250°F and 300°F, I will let it run. If I am short on time, I will adjust to 300°F.
7. do not open or mess with the brisket for at least 4 hours, as long as you are under 300°F.
8. Go out and probe it, if it is tender, remove, if it is not, leave it, return in another hour to hour and a half. Never probe the point, that means nothing. Only probe the flat, at the thickest part.
9. repeat until the flat is tender. Never under 250° or over 300° except for special situation.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"I'm Armenian...perhaps...but then again...maybe not..."