One thing to keep in mind is that there's no such thing as rare, medium, well when it comes to BBQ. You're cooking well beyond those temps in order to break down the connective tissues and really tenderize the meat. Ironically, undercooked brisket, ribs, pork butt, etc will have a similar texture to traditionally overcooked meats when you're cooking to traditional temps. It's cooked thoroughly, but you haven't broken down those tissues...leading to it still being tough and dry. BBQ tends to become a fall-apart mess when it's overdone (fall off the bone ribs are a sign of overcooking).
I'd recommend using the temp only as a rough guide. Use probe tenderness and feel the looseness of the meat (ribs and brisket) to gauge when it's truly done.
Also, you're cooking flats by themselves. As you may already know, that's the lean section of the brisket - if you're also getting cheaper/sale cuts from grocery stores, they're going to be choice/select cuts and even leaner on top of that. Next time, bite the bullet and grab a prime full-packer from Costco, let it sit in your fridge (in the cryovac) for 10-14 days, and then cook it. The difference will be significant.
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