Based on your last post I think probe temp accuracy is fine (tick that off the list lol).
ambient temp seems fine since your cooking times are roughly what your expecting.
Im thinking radiant heat from the deflector plate being much higher than the ceramic sides and lid's radient heat.
3 things I could think of is:
1, raise the cooking grid higher to get away from the radient heat.
2, bend more angle into the 'folds' of the deflector plate so it sits lower/further away from the cooking grid.
3, Similar to pic 6 but maybe invert the drip tray to act as another heat shield.
I once was cooking some veg indirectly and wanted to put the garlic bread in aswell, so i put them on my drip trays....which were in direct contact with the deflector plates...great big dirty black incinerated line on all of them that tasted awful! and the butter wasn't melted haha. Anyway your experience reminds me of this and thats what is making me think of the radiant heat๐ค