It almost sounds like you just may have had a tough piece of brisket. I usually do at 200-225, but 250 is certainly within reason. I don't flip, but I do tend to wrap in butcher paper after the stall, and that helps with moisture, though somewhat at the expense of bark.
In terms of the meat quality, I use either choice grade, or on occasion, prime, so that may have played a role in in not coming out the way you had hoped. I also wouldn't necessarily consider brisket, pull-apart tender (at least not the flat), but in theory if you took a piece and draped it over the knife, it should bend easily without breaking.
Wish I could give you a magic answer, and hopefully the truer experts here will be able to give you more :)