You could try an injection of beef broth.
IMHO, it’s the size of your brisket, which is probably a lean flat cut. Small flats are tough to keep moist for even experienced bbq cooks. I have switched to cooking only packer briskets, which is overkill for your purpose. Try to find the point portion of the brisket, which is fattier and more forgiving to cook. Alternatively, go for a well marbled chuck roast, which has a brisket like flavor and is very forgiving.
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