Matt, per Travis, foiling at 160F or even lower will eliminate the stall and reduce cooking time substantially. Assuming 225-275F cook temp @ 5.5lbs, you can get away with starting as late as 10am as long as you foil between 140 and 160F. When you foil, add some beef broth to the package.
For rub, I would use something heavier than a butt rub. You can search for beef rubs here. The flat has less connective tissue than a packer so you need to keep more of an eye on tenderness than temp for finished state. Use a skewer if you have one to test by inserting it into the thickest part.
When done, use the package juices to create a sauce/jus/gravy and apply it to your sliced meat when serving. I serve a jus on the side.
If your want a more firm crust, unfoil around 30 ninutes before doneness and put the flat back on dry heat. This can be the smoker, or easier, a dry oven at the same temp.
Brisket seems to be one of the hardest cuts to master but it's still low and slow BBQ. It will forgive as long as you don't kill it.
Worst case, if you aren't happy, toss what you have in a crock pot with some root veg and liquid and you will get a great smoky day after stew. Add some potatoes or serve over rice and it will still be something you can be proud of.
Good luck !