Michelle, your first assumption is correct.
You must be an extremely competent cook to select a beef brisket for your first SV cook. You likely know briskets are one of the most challenging cuts of beef to cook successfuly. Most cooks attempting a new technique start with a less challenging and smaller piece of meat. You are one brave person.
As you know briskets have a natural separation between the Point and Flat parts. Use it to guide your cuttingthe brisket in two. The you can make your choice which one will be cut in half again for your third piece.
That mass of meat will displace abou a gallon of water. Therefore you need to have a vessel with sufficient capacity for ample water circulation.
It seems to me just about evryone has a personal specific doneness preference for brisket and they vary widely. Your brisket will be tender and Medium-Rare.