It sounds like you did everything right. Liquid in the drip pan is just to create a heat sink to regulate the smoker temp; contrary to popular belief, the steam adds nothing to the moisture of the meat; the moistness or lack thereof is more about how much fat remains. I used to get all fancy with the water in the pan, adding a gallon of apple juice or a bottle of wine to ‘flavor’ the meat. Didn’t make a darn bit of difference. Like @pulykamell, I’ve used sand for a heat sink for years and it doesn’t negatively affect the final result at all. And makes for much easier cleanup.
I think it’s just that 5lb. flats of brisket are notoriously finicky to cook just right; nothing so much that you did wrong-- they can go from ‘too tough due to improperly rendered collagen’ to ‘dried out jerky’ in a short window of time.
Maybe try a full brisket next time? I post this link of mine from last year every time somebody mentions brisket in a thread, so I hope I’m not getting obnoxious with this, but this experiment I tried cooking a full flat + point brisket on a kettle-style grill turned out so well, I gotta evangelize a bit. I need to try this again soon: