Good morning, the reason your brisket was a little tough, is because it was not cooked long enough. Brisket is different than steak because that are from different types of meat entirety . Brisket is a tough piece of connective tissue and muscle that requires a long cook Low & Slow. I am one of the old school (old timers) that believe if Done when its Done. Briskets are different, you can take briskets that are almost identical smoke them together and most every time one will be a little more tender or juicier.On a small flat I wouldn't smoke over 225º if its very lean wrap early but that internal temp needs to be higher than 160 - 175 I would go 185 up. I did one smoke a while back and posted pictures where I actually checked my brisket temp just for the post. I have cooked so many briskets I never check the temp I can tell by feel and looks. Now I do realize this does take years and you are starting out, but do not rely strictly on temp when smoking brisket. Hope this helps
Gary S