Yes, meat is done when it's done, not based on time. A faux cambro, whether it's a cooler or just a microwave oven, can keep the meat warm for quite a while when wrapped in a towel. The ice cooler works better of course, but putting wrapped, cooked food into the microwave to store is still better than letting it sit on the counter wrapped up, since the microwave is an enclosed box.
With one that small I'd have everything going just after lunch, even if dinner's not until 6 because brisket can take awhile and because faux cambro.
You really have to develop a process to cook, and then practice by cooking more food.
I never cook anything often enough to get to that point, and my logs aren't adequate guideposts. I have some intuition, and Dominoes number keep handy.