I think Smokin would tell you that the temp swing doesn't bother him. Try moving the Maverick probe to another location and see what it reads.
Gee. I hope my butts don't take 20 hrs. Otherwise I'm ....... Normally, a 7.5 lb butt doesn't take me more than 12 hrs (Elite).
You know. I don't really know what the temperature swings are on my Elite. I've noticed a few degrees one way or the other. I just watch the Elite's meat probe and the Maverick probe readings. Kinda surprises me in a way cause I'm normally anal about those type of things.
Just a thought. Maybe your Smokette reads higher than actual, and when you set the Smokette lower you're getting longer cook times. Verify the accuracy of the Smokette with a verified accurate oven thermometer. I've also dangled the Maverick probe inside my Elite and it read the same as the Elite oven temperature.