Depending on area and height of the piles, 3% error with that GSD would be quite alright?
Anyways, if your DEM GSD is 5.88 cm/px, you won’t get anything out having vertical accuracies under that value: https://support.pix4d.com/hc/en-us/articles/202559219-Error-estimation-in-volume-calculation
However, if you process your DEM to your original GSD, then it makes more sense.
Also, using network adjustments and observing each point multiple times will greatly increase precision to agree where millimeter-precision using gnss is possible: Getting sub-centimeter precision with Reach RS2 and EzSurv
From your image I can’t see if it is a deep open mine, or relatively flat? If deep, you might run into trouble obtaining enough sky-view, in which case (much) longer obs-times are needed.
https://community.emlid.com/t/using-4-x-rs2s-for-precise-results-in-challenging-environment/24076/5