Welcome Brian, and congrats on your #2! I too came from a long WSM background. So far, I haven't used my WSM since I bought my SI. For some reason, when you buy flats alone, they tend to be "over-trimmed" of fat, probably to appeal to the average consumer who is afraid of, or doesn't want to pay for fat. Too much trimming can get dry, no matter which smoker you use. Looks like your flat has a little nugget of fat left on it. Try a whole brisket next time (and trim it yourself), and I think you will find it to be more moist. Leave at least 1/4 inch of fat cap if you are trimming it yourself. Costco has Prime whole packer briskets at a good price.
The internal temp will rise quickly in the beginning as you noticed. Lots of people find this alarming. But it will stall. You did the right thing wrapping in foil and putting in the 300 degree oven. I've done this several times when I was in a pinch. The lack of a good long rest I'm sure has something to do with the tougher than usual meat. I'm getting to where I allow lots of extra time for the cook and the rest. I've rested up to 6 hours (double wrapped in foil, 2 towels) in a cooler, and the meat was still hot.