It took a while to mentally compute, validate and recompute solutions, but I've got a solution for the belt path that will allow for all axis motors to be stationary, cutting down on moving mass and cables. It's similar to the roller but the belts are not on the same plane so that they could be successfully routed in the space available. I plan for the extruder to be on the arm due to the flexibility of the fibres.

I was wondering when I'd have a reason to use this exotic looking motor:

These two motors are responsible for the 2 revolute joints and a 3rd motor will be on the other side that will move the arm across the X axis.