I dont cook brisket that way, but that shouldnt change tenderness? IMO... A couple of things :
1) flats can be very tempramental and dry, really better to do packer and trim down.
2) the rest time seemed a little short, as the meat rest it will reabsorb some of the juices that rendered out of it...
3) you might also try(if you have a grill of any kind is to sear the brisket first then put in oven and cook....
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