Rule of thumb for brisket is hour to hour:15 per lb at 220. Your 5lber went 10 hours(2 hours/lb).
A 5 lb brisket sounds like a market trimmed flat. Assuming thats what it was, your times were definately on the long side. How thick was the fatcap on the slaB? id it was thin, you gotta lay some bacon or porkfat over the top. When u remove it, the internal temp of the brisket should be no less than 195.. i take mine to a few degrees over 200 internally, then towel wrap it and cooler it for a bit. Also, what were your expectations? A flat will never get buttery tender like the nose and point, it will always me a little more firm to the teeth.
Also, 2 cups of mop is a bunch!! When i wrap mine, i spray it down enough to be good and wet, but it sure aint swimming. Is it possible that you were overcooked and dried out?
When your cookin, you can do the fork test. Stick a for in it and twist it, if it twists easy, its done, if it puts up a fight, you got more to go.. but internal temps. Take out at 195-205, wrap in a towel and let it rest for an hour or 3.
hey... keep this in mind... for your first cook, you took on the most challenging hunk of meat. Birds and butts are more forgiving. Took me over a dozen full briskets probably close to 2 to get it to braggin rights. If ya really want to try a brisket again, try to get a full packer cut. Those trimmed points dont give you a lot of leeway.