So two things, I personally think flat only should be saved for Jewish grandma brisket. It’s just better suited to braising when it doesn’t have the point to protect it and share the thermal load. Second, when you pull the brisket at temp, you need to let it sit out for 45 minutes to an hour before putting it in the cooler. That brisket may have been perfectly cooked when you pulled it off, but you have to let it stop cooking. 225 is fine for brisket and a lot of people cook them a good bit hotter than that with good success. That being said, don’t be too hard on yourself, brisket is hard to nail and those who do have probably turned out at least a few bad ones getting there. It’s a shame it’s such a big piece of meat to experiment with.