Small briskets, especially the flats, are always a challenge to keep moist while smoking. I too have trouble with them but over the years have learned that they must be done low and slow...no higher than 200F-225F,with a water pan under it, preferably right on the flavorizer bars to help keep it moist, with frequent spritzing (cider vinegar and water or cider and water) to the stall (around 155-160) then tightly wrapped in butcher paper until āpeanut butter tender) around 200F. Finally it then has to be tightly wrapped in towels or blankets and placed in an empty cooler to allow it to re-absorb the moisture.
Alternatively, buy a whole packer brisket, trim, smoke as above and then vacuum seal the meat into future meal portions, and freeze.