Yeah, the controller part has already been investigated and solution found. 
Going with g2core (the official successor to TinyG) for software, running on an Arduino Due with a custom made stepper board. The custom made stepper board wasn’t actually all that hard, once I figured out the right approach. Doesn’t need anything super special, as it’s not a “production run”. Instead, just manually soldering the right bits onto perfboard works fine. Tested already. 
Fusion 360 already generates 3 axis stuff for g2core (using the “TinyG” post), and in my initial testing to add 4th and 5th axis capability to the output, that’s been pretty easy too. The post processor parts for Fusion 360 are mostly just a bunch of toggleable options in javascript (Fusion 360 posts are javascript), so copying the right bits from Tormach/HAAS/etc posts has worked fine.
I need to circle back and get the mechanical bits sorted next, to get a full end-to-end iteration working. But, some stuff has come up so it’s been delayed. 
