This is a brisket flat I did in the past, only one I have access to pics of right now.
You don't indicate how long your flat cooked, but that small of a flat could be part of the problem. I cook flats all the time, most average 6 pounds, some have been smaller, some larger. I start by trimming some (not all!) of the fat, then season with kosher salt and black pepper, only. I start it on my Traeger 075 fat down to smoke for 3 to 4 hours. The temp setting on my digital may need to be 180 to 225 depending on weather. When the flat gets to around 165 degrees I will put it in a foil pan and cover with foil without adding any liquid and put it back on and turn up the temp to 300 to 325 and cook for another 2 to 3 hours (until it probes easy). The meat temp is usually around 203 when I pull it off. I leave it in the pan (the pan is full of juice) for around an hour (no oven, no cooler) then slice. I always make 2 or 3 at a time and Foodsaver them and freeze for later. This works great for me, others may do it different and have great results also, that is the beauty of BBQ to me, there is no 1 way of doing something.