Thanks for the shout-out and congrats on the oven!
If it was me, I'd do firebrick splits under the saputo tiles. My floor is 2.5" (6.4cm) thick, walls and dome are 4.5", and heatup time for me is minimum 3 hours. But, it stays hot for days. We cook in the oven up to 3 days after firing. Detroit-style pizza the day after firing. Even day 4 we're still around 200ºF and we do crock-pot dishes and oven-dried tomatoes. The extent to which we're able to use residual heat is a surprise and unexpected benefit. It's so good, my wife hasn't voiced a single concern over the cost of wood, which believe me, is out of character for her.
My vent diameter is 6" and it's fine for the 36" oven. I draws great once it's hot, and a little smoke out the front when it's cold, which I think is normal regardless of vent size. It's why I used black firebricks on my outer arch. I think the prevailing wisdom is that 8" is ideal, and you can get away with 6" for 36" ovens and below. I'd stick with that advice.
My metal was 10 gauge steel, which I think was good for the arch and front plate, maybe overkill for the straps, and definitely too thick for the door. My door is quite heavy. It's probably the only thing I'd do differently.