[dmun]

As you know from running your oven, the top of the firing chamber burns white first, and it takes a long while for the carbon to burn off the sides. I suspect that your buttressing might exacerbate this problem.

I'm thinking that having brick buttresses, at intervals, with the door opening functioning as two, and maybe four others to carry the stress of the dome weight to the floor, without adding to the overall mass of the oven. Cathedrals are built like this, with stout columns bearing the load, leaving space for windows and "curtain walls" in-between.

Forget about perlcrete having any tensile strength. Even fully cured you can dig your fingernail into it like cork.