Yeah there's a sweet spot.
I try to aim for about 120⁰C which is about 150⁰C on the probe built into the BBQ.
Too short a time & the connective fibres won't break down enough.
Too long uncovered (> 6hrs IMO) & you get dry meat especially in the bottom near the heat source.
I find transferring it to a dish after 5-6 hrs and placing a cup or so of water in with it makes it pretty full proof. Covering it is a good idea but the water stops it drying out. Water keeps the base from going over 100⁰C and keeps it moist. Too much water & the brisket losses it's flavour (a common problem I've found with restaurants).
The only thing to watch for is that the water hasn't evaporated. A good brisket has a lot of fat that renders & pools in the pan. It looks like water but gets a lot hotter. I sometimes transfer to an oven to finish which makes things a bit easier.