What are you using for layer height? Are you using PETG?
I suppose drawings for the hole locations are unavailable, but....
FWIW, I've had great success 3d printing jigs for drilling holes in locations specified in drawings where I need to do a couple of sets and doing it on the mill for each one would take too long. Generally, I set the diameter on the 3D model to drill diameter. When it prints, hole is slightly smaller which I clean out with the proper sized drill. I make sleeves on the jig, the better to align the bit and do use a drill press.
I also wondered if the design for these parts would work if the center was left mostly open, say using 3/8 inch rim around center which would be enough to trap this part between the face plate and whatever is behind.
Thanks for giving this project the care you are.