Thanks for the advice, Zach 
Thankfully I haven’t turned it on yet and have merely been assembling on the carpet. I actually went and built a torsion box out of 0.5" MDF with 2" tall core pieces. I’m going to do @IanWatkins mod to stabilize the machine and flatten the work area by bolting the wasteboard, on top of two more pieces of 0.5" MDF, straight through the torsion box with a few 0.5" diameter 5.5" long bolts. I’m hoping that kind of stability will enable me to reduce runout to the point that PCB milling will be achievable without much tweaking after it’s dialed in.