What temp was your cooker during the last 30 min of cooking?
What made you pull them at those temps?
How long did they rest and were they well wrapped?
At pulling ,did the meat fall apart, pull apart very easily or...?
As for future cooks, I would suggest not buying butts at Costco, or any other place that only sells them boneless. They hack the meat like first-week novices. Inconsistent size/thickness means inconsistent cooking unless you tie correctly first. Why bother? Get better butts elsewhere - bone in.
Mopping is unnecessary. Save yourself the trouble.
I am not a clay fan myself but it can work just fine. Clay instead of water - or nothing instead of water - can sometimes push the finish of the exterior portions of meats toward dry if the temps are on the higher side and/or the meat doesn't get flipped/rotated.
Finishing sauce (I prefer
this one) can even texture, moisture and flavor between the various muscles a butt comprises when it is pulled/chopped, thus mitigating overcooking problems were any to have occurred, but it is a good idea to do whatever you can to avoid undercooking or overcooking from the get-go.