Okay, got it.
Before I started smoking meat a few years ago, I roasted pork butts in the oven at 350F. I did so for decades. I rubbed them, then started them in a covered roaster for a few hours before removing the cover until they were fork tender. They always came out juicy and delicious with a nice bark.
Once I started smoking meat, I read about low n slow, believing something magic happened to the meat. It didn't. It just took longer. I've since smoked butts at temps from 225F to well over 350F and always got the juicy, tender result I wanted. How quickly I needed the butt to finish really drove my choice of chamber temp. So, throw chamber temp out the window for the reason you have a tough butt.
What you need is experience in smoking butts since this was only your third. Technology is a fantastic enhancer for experience, but it can be a major distraction while trying to learn. So, since butt's can be had for a fairly inexpensive price per pound, try an experiment, and I'll bet you'll end up with a juicy, tender butt that you'll be proud to serve. Do the experiment when you're not up against a clock.
Prep your butt like you always do. Fire up your Kamado in a way that it preheats slowly, like 2 hours or more. A quick fire is hard to control. A slow fire is easy to control. Let's pick a smoking temp of 275F just for experience' sake.
Load your meat when your smoker stabilizes at 275F, and don't use any meat probes. Don't sweat the chamber temp drop when you load the meat. The fire is still burning at 275F and the meat is absorbing the available heat. Now, here's the experiment. WALK AWAY FOR ONE HOUR PER POUND. Don't peek, spray, wrap, or do anything to open the smoker. Don't make any chamber temp adjustments unless you see a 50F difference from 275F, then make them small. Due to physics and chemistry, a butt doesn't care about the chamber temp or the swings. It only matters to the clock.
After 1 hour per pound has expired, probe the meat for tenderness. Take it's temp if you like, but learning the feel of the meat is your purpose. It is practically impossible to overcook a pork butt, but it is SOOOOOOO easy to undercook one, resulting in a tough, dry, chewy texture.
If you feel resistance when you probe, walk away for another hour. When the probe slides in easily, with little to no resistance, you're done. Take it's temp at that point, purely for information's sake. Then remove the butt, cover it, and let it rest for an hour or more. Shred and serve.
Now you have an experience baseline for understanding what your technology is telling you. A butt LOVES to be forgotten about when it is smoking. Do it and yourself a favor, forget about the butt. Folks love to say "looking ain't cooking." And they're right. But thinking about a butt, the stall, unexpected meat temp drops, secondary and tertiary stalls, is the recipe for smoking insanity. Take all that out of the equation, and you'll be happier with the result.
I have an obsessive analytical personality, using technology every chance I get. But some things just turn out better when you use technology to enhance raw experience and understanding, not create it.
Ray