That is one of the greatest draw-backs of the MES30, briskets and ribs just don't fit. BTW you can coil the ribs and they fit and cook fine in the MES30 whole.
I do the same thing with brisket, I trim across the flat, exposing the grain so I'm able to slice without thinking when done. The corner cut is usually about 3 or 4 lbs, I just grind it for chili. No better chili meat I know of unless your a butcher.
Sounds like you have a handle on it. Just remember the old adage, stick a fork in me, I'm done. time /temp is only a guideline, as mentioned above probe the meat when you are sure its done.
Allow to cool before attempting to slice or you'll have chopped brisket......
YOU know what to do, now its all about time and patience and leaving the door shut.