Can I rest a brisket for 30 minutes?

You can, and the flat will leak on the board. Two hours is the short plan this site uses, the low end of a 2 to 4 hour hold. Slice when the center has come back toward about 150°F, not while it is still roaring at 200°F.

How do I keep it above 140°F?

A dry cooler, the wrapped brisket, and a towel slow the drop. A cooler is not a warmer. If the center is heading under 140°F and you are not ready to slice, hold it in a low oven or on a quiet pit, not in the danger zone.

Does pork butt rest this long?

The pork plan sets aside one hour to rest and pull. Brisket wants the longer hold because the flat punishes a hot slice more than pulled pork does.

How long to rest brisket

Rest a brisket for at least two hours, and keep the center at or above 140°F the whole time. Two hours is the short end of a 2 to 4 hour hold. Slicing as soon as it leaves the pit pushes the juices out of the flat and onto the board.

Why the rest is part of the cook

A hot flat is tight. The juices are moving, and a knife opens a path for them to leave. As the meat cools a little, that movement calms down and the slice stays moist. The bark also firms instead of steaming the cutting board. None of that happens in the twenty minutes it takes to carry the pan inside.

The brisket calculator already includes a 2 hour rest and about 45 minutes of preheat in the fire time. If you built the plan and then skip the rest, you don't get those two hours back as free time. You change the texture, and the juices end up on the board. The planning finish is still around 203°F. Rest starts after that, which is brisket internal temp.

Pros

A two-hour hold gives an early brisket somewhere to go besides the trash talk of a late dinner.
The flat slices cleaner, which is the difference between a platter and a puddle.

Cons

You have to watch 140°F. A cooler only slows the drop. It will not hold forever.
Wrapped rest keeps softening the bark, especially in foil. Vent the foil at the end if the crust went soft.

How to hold it

Leave it in the wrap. Set it in a dry cooler with a towel under and over, and put a probe in the flat so you are not guessing. A Meater Pro or any leave-in from meat thermometers for smoking can do that job if it still has battery. If the center falls toward 140°F, move the brisket to a low oven, around 150 to 170°F, or back to a quiet pit. Do not let a long hold live in the danger zone because the cooler "felt warm."

Slice across the grain when you are ready to eat, not an hour before. The brisket rub is on the surface you just spent all day building. Give it the rest, then cut what you will serve.

The stall and the wrap decided when you got here. If you are still in them, read the brisket stall and butcher paper vs foil before you worry about the cooler.