Ok, here's the verdict. It was overcooked. The flat was pretty crumbly. Either I should have pulled it off the pit sooner or it overcooked from carryover heat in the cambro, or maybe it was a little of both.
All of the flat was probe tender after 8 hours except for the part about 4-5 inches from the end so maybe I should have pulled it then, but who wants a tough brisket in that area? Maybe I should have let it sit on the counter longer before I put it in the cambro or never put it in the cambro at all. There was only a couple of hours between the time I pulled it from the pit and the time I served it.
Also, I trimmed the first brisket I cooked pretty aggressively which I thought might have contributed to its dryness so I probably didn't trim this one enough. The fat cap on the point rendered down really well, but the fat on the flat hardly rendered at all which resulted in me losing some good bark on the flat slices. I will trim the flat down more on my next one.
My guests loved it, but they have never had good brisket before. The flavor was good, and the bark was awesome. Nobody could believe that I only used salt and pepper for the rub.
Overall, I was pretty frustrated with the whole thing because I am a perfectionist, but this only made me more determined to get it right the next time. I will probably try Bludawg's method on the next one. These darn briskets are a lot harder to cook well than a pork butt!
Thanks again to everyone for their help. I really appreciate it.