Adding some butts will help - more mass and more heat coming in later. I commonly leave butts in a cooler for 6-8 hours when I do overnight cooks at home, then don't mess with them until dinner time.
Leave a probe in a brisket and keep an eye on it. If it starts getting close to 150, warm up the oven and use it to hold. But I think with a few butts, the cooler should be OK.
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