Problems cooking Wagyu brisket

Your brisket was undercooked, not overcooked. At the temperatures you stated, rule of thumb for brisket cooking time is about 1.25-1.5 hours per pound. Convection may drop that down a tad. A 7.5 pound brisket should be cooked for at least 8 hours, and could be closer to 10 or so.

The cuts of meat that are designated as "barbecue" meats are generally full of connective tissue. You put it through a long, low-temperature cooking process to render down that tissue, which will both tenderize and moisturize the meat. Undercooked barbecue will often be mischaracterized as "dry," because the amount of chewing required on the meat robs the mouth of saliva. Properly-cooked brisket will slice easily, and a slice should easily pull apart when gently tugged with two hands. This is not a steak, so the sensibilities you would apply to cooking a steak go out the window. A brisket has only just BEGUN to break down its connective tissues when it reaches what would be regarded as "Well Done." It is not likely done from a quality perspective until the internal temperature of the meat has reached at least 190f.

Next time, budget a cooking time 1.5 hours per pound. If it is finished sooner, wrap it in foil and towels, and leave it to rest in a cooler. It will safely keep that way for up to four hours, and a long rest will result in a better product anyway. Start checking the internal temperature of the meat about halfway through your budgeted time. At around 165f, wrap the brisket in foil, pouring some beef broth into the wrap, and return it to your oven. Start checking for doneness at about 75% of the cooking timeline you have laid out. You should be able to slide a probe in and out of your brisket easily, with just a small amount of resistance. That is when you know it is done. Not by time, not by internal temperature, but in the tactile clues provided by the meat. You can optionally take the brisket back out of the foil when you think your are close to being done, in order to give yourself a better bark. When you are confident the meat is done, rest in a cooler (using the aforementioned foil/towel treatment) for at least an hour.

When you are ready to slice your brisket, be sure to SLICE ACROSS THE GRAIN. This will serve as another tenderness aid. You should also reserve the jus that is generated, and pour it over your brisket slices. This should enhance your success with making a brisket.

Wagyu it not anything special comparing to other grass feed cattle, it's just more fat marbled. So if you like that, then it's great

The increased marbling is mostly in the top cuts, and not so much in the bottom cuts like brisket, so Wagyu cross breed cattle isn't going to make that much difference for this

Your technique seems overly complex! Just cook it, covered, until done by measuring the internal temperature