Your setup is almost identical to mine except mine is 18.5" and I do not use the same "heat diffuser" on the charcoal grate, but instead I wrapped some HD foil around the charcoal grate (on the 18.5" it is about 16" in diameter) and placed this directly below the tile.
I have some of the same issues as you - too much char on the rim and our crust & crumb look identical, but, unlike you, I actually wanted a bit more char in the bottom-center, so I actually cut a round hole in the center of my makeshift diffuser and this seemed to help, but I cannot cook a pie in less than 3.5 minutes w/out over-charring the rim- I'm actually pretty happy w/ my 3.5 min. pies, but I'm still fine-tuning my technique
The point that several have made about the different types of flour is a good one (I currently use about a 50/50 blend of good quality AP & BF - I haven't converted over to 00 yet for a # of reasons, but maybe someday) - but if I recall, our founding father Villa Roma & others were able to get great results w/ AP, so maybe it also has to do w/ variety w/in the brands or maybe there's other factors at play? - based on the pattern that it chars, I'm pretty convinced that the "jet-stream" of hot air coming up over the perimeter contributed to the increased char on the rim - so, contrary to some others, I actually blast the burner just before I put in the pie & then back off a bit once the lid goes on & this gets pretty good results
A couple other notes:
1. Sounds like the tiles have already started cracking - mine have as well but they still work great - especially for $1
2. As the veterans here pointed out to me, your main grill will probably start to sag soon - especially since yours is 22" - they have posted some great solutions, but a quick and cheap one that I used was to reinforce w/ a couple rods of rebar - just drilled a few holes lined up along the sides and threaded the rebar through - make the holes only slightly bigger than the rebar and it will mechanically lock in - so far this has held up very well