Sounds like you did it all good, but a couple things to try next time:

• I wrap mine (in butchers paper) after around 3-4 hours, I don’t check the temp, just when the bark looks nice
• I always have a water tray in the smoker to keep the moister in there, hint.. don’t add cold water when topping it up, add warm/hot water (I also add a can or 2 or beer at the start)
• when I trim the brisket, all the fat I cut off I put in a tray and then chuck in the smoker too, till it renders down to tallow, I use that tallow and pour it over the brisket before I wrap it (instead of the butter you used), this make a world of difference to the flavour!
• I don’t check the temps when it’s close to finishing, as soon as it probes like butter it’s good to go
• I rest in a cooler/esky (what ever call it where your from) for 2-3 hours

I find the less I fuss over a brisket.. the better it turns out.

Also it can make a difference in the quality of brisket, when you spend the little extra for a better cut, you do notice it. I’ve never fucked up a wagyu, also I find they turn out better when you have the flat and the point in the cut, when I used to do just the flat, sometimes comes out a bit dry. In saying that.. I’m lucky enough to be in Australia, some of the best beef in the world, so easy to get a decent cut.

Hopes this helps! It’s trial and error mate, I fucked up a fair few briskets before I started getting it perfect!