Figured out why the arm was so flexible: Cura was only slicing with 2 walls instead of 4. It appears to be some bug in how I configured the Deltesian as a custom machine. I created a new machine type in Cura, set it as a Delta Kossel Mini and then set the bed size to 200x420mm and Cura sliced the model with 4 walls as intended.

The 4 walls are important b/c (1) the holes for the bearing are designed to be solid plastic when printed, i.e. no infill & (2) those layers are what give the arm beam stiffness, like an I-Beam.

FEA testing in Inventor

I need to get a better idea on some of the failure modes of a shape like this arm so I running some analysis of the arm under different loading conditions.

It's Important to note: