While I’ll be getting some more expensive setup, I am going to try a budget-friendly, non-committal solution that will allow adjustment.
A bolt threaded through the bench-top right under each of the supporting steel planks making the base of the Shapeoko should allow easy adjustment of sagging with a twist of the wrench.
I figure two bolts per plate for a total of six should allow me to make fine-enough adjustments. Only add another nut right before threaded plate to keep bolt in place…