Just a suggestion, so take it or leave it........
.......before you go all out analyzing & modifing the cooker, give it one good try operating it as is......
Set up to cook & do a practice run with no meat......just a pan of cold water on the cooking grate to simulate a lump of cold pork going on.....
Get the airflow running real good, just use a dinky $4 oven therm on the cooking grate.......
Start the burn.......vents mostly wide open to start & all 3 bottoms to 1/2 open after 20 minutes.......
TOP ALL OPEN ~~> The fire needs to exhaust cleanly.......
30-40 minutes in, check the therm & be aiming for 300f....adjust a little from wherever you are to get there....
Even try it with briquettes, just make sure they are bone dry & not from a bag that's been exposed to moist air......throw some wood chunks in, buried just like you would for a cook...........
The idea is to get good strong running start, THEN add in the cold piece......after the initial adjustments, NO fiddling.....NONE.....just see how it behaves.....leave it basically alone to do what it does......
Maybe the longest butt I've ever done needed a lot of extra coal added......15 hours.....in driving rainstorm that was unexpected.......
But, once I started, it gonna get done......
No Pig Will Ever Get The Better Of Me !!!
Coal coming up on once around.....
Next morking when the rain broke, dead on 250......
Finished piece going into hold.....
The point being is to get a good solid burn going & keep it as stable as possible.......not easy in cold wet weather, but all the extra blowers & therms & head scratching & coal changing in the world won't replace a good solid burn @ the start.....