You saw the temp drop after putting it on because the brisket was a huge heat sink. That’s perfectly normal. To avoid the temp from going too low, let the WSM go a bit higher in temp before you put the meat on.
If the temp drops later in the cook, make sure you tap the bowl to knock the ash off periodically. The excess ash, which kbb is famous for, will impede airflow. That should help. If you’re pondering an atc, they are cool but not necessary. Briskets are just a bit more finicky to cook.
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