Hi Cal and welcome to the permies forums!
Round rooms are always more difficult to build using straight exhaust pipes.
The heat extraction area of rmh's is often not built with the thought of inducting extra friction to slow the gasses down but rather to allow the gasses to naturally stratify through the use of bells.
This is often a much more efficient way to capture a lot of the heat before it exits the chimney without sacrificing draft.
Bells are also not restricted to shape and size. If you don't have the floor space, you make them tall. If you have a creative design, they can be made curvy and spread out through several branches.
Lots of possibilites to consider.
wood-heat-storage-flues-vs-bells