Here’s a mockup of the vent arch. The lower part of the arch is made up of 9-inch firebrick, split the long way to make square-ish blocks. The arch is bridged by 1-inch pieces, leaving an eight inch opening at the top. The wooden box represents the flue liner.
The arch only has to support for feet of flue liner, so hopefully I can get away with the puny voussoirs across the top.
This arch will be insullated with perlcrete, and the adobe bricks will surround the insullation, forming a type of abutment to support the arch. The chimney will either be insullated with perlcrete covered with stucco, or will have an air gap, and then stucco supported by metal framing.