My thoughts are: (others may vary)
You need 2 butts. (unless you have plenty of sides to go with them)
I never spray/mop my butts.
Do not brine or inject them, they are very moist without it.
If I use mustard I will apply it first them cover with my rub, others do it the other way around.
Another thought, make sure to cook the butts till they probe like butter, this should happen between 195* and 205* internal temp.
Give yourself plenty of time for them to cook. They will stall around 160* - 175* for a few hours depending on what temp you are cooking them at. The butts will need to rest in foil at least an hour after cooking before they are pulled.
X-Large Big Green Egg