When to slice the brisket?

Definitely chill before slicing and always slice against the grain. I also keep meat in sauce for 2-3 days before final reheat!

I let it cool to warm or room temp and slice before chilling. I find it hard to slice when chilly from the fridge.

Definitely chill first, then slice and reheat. We separate brisket and sauce and chill both, removing fat from the top of the sauce and then pouring over sliced brisket and reheating.

I have always sliced it once I take it out of the oven and then refrigerated it. However, now that I am thinking about it, it may be easier to slice once chilled.

We always have sliced it when cold, before reheating.

Remove the pan from the fridge and take off all the fat. Then take out the meat (scraping the sauce off carefully, so you lose as little as possible on the cutting board. Slice the meat to your preferred thickness (not too thin) and return it to the pan. Cover and heat in the sauce.