The 5th axis build came together surprisingly well. I had expected much more resistance getting the unit to be known to both fusion360 and LinuxCNC. There is still some tinkering to be done for sure but I can get some reasonable results already. The video below gives an overview of the design:
Shown below is a silent movie of a few test jobs I created to see how well tool contact would be maintained during motion in A and B axis while the x,y,z are moved to keep the tool in the right position. This is the flow toolpath in Fusion360 in action. Non familiarity with these CAM paths makes for a learning curve which is interesting when paired with a custom made 5th that you are trying to debug at the same time.
I haven't tested how well the setup works when cutting harder materials like alloy yet. It is much quieter and forgiving to test cutting on timber and be reasonably sure about the toolpaths and that you are not going to accidentally crash to deep into the material after a 90 degree rotation.