Probably too late but best to cook and then the next day, slice what you need, seal in seal-a-meal or double ziplock bags and drop in a pot of simmering water for maybe 10-15 minutes. I’m guessing cooking part way will confuse the meat fiber and you’ll have a rock on your hands. There’s plenty of net info in this, look at Malcolm at Howtobbqright.com to start.
My preference would be to cook overnight and shut it in a chambray (a good cooler) wrapped tightly in foil. It’ll keep six hours or more, maybe throw a couple of towels soaked in boiling water in first to heat the cooler. Most bbq joints will serve brisket as much as 12 hours out of the smoker although they do have fancy holding equipment to keep it nicely.