Mike is correct. I have done this type of work several times. I have a client that wished to have 400 acres scanned and mapped to the nearest level of accuracy. The area was a rock quarry just as you seem to have encountered. After mapping it with 80/80 overlap, I was able to get 5ft. of accuracy. I have a fried who is a survey professional and we discussed letting him set some GCPs with his 30K survey equipment which I gladly accepted. After remapping the same area, the accuracy was 3cm (about 7 inches) which, as he explained, is survey accurate. I went back a third time with a handheld GPS device, placed it in the center of each marked GCP measured by the expensive survey equpiment, recorded the lat/lon/alt. and at each GCP, could already tell the handheld position was moderately different. I then flew the quarry a third time with the handheld GPS information and the accuracy was 3ft. So, if a client wants survey grade accuracy, using a professional surveyor to set the GCP is the best way to give the client what they want. In the end, you will pay the surveyor for his work but only for the first time. Every time after that, you already have the information for the subsequent flights.
Interesting results on calculating volumes based on a comparison of same stock piles from each of the three flights . . . . measurement differential average was just shy of 1%. This, I believe, shows how good DD is at processing viable, usable data for less cost. The cost for a surveyor will have to be passed on to the client but if your results are +/- 1% from results of drone mapping, then I'm not sure what the need of centimeter level accuracy is at that point. But, if that is what your client wants, that is what your client should get.
If you want to demonstrate to potential clients (mining/quarry), once you get one producer to agree to a demo, it would be worth hiring a surveyor to set your GCPs, fly the area and then go back and fly it without the use of GCPs. Then compare the two as it relates to volume calculations, slope grading, etc. I believe you will see that the difference will be minimal. If that is the case, show those results to the client and let them know that if they want their other assets mapped (lots of quarries are owned by corporations; therefore they have more than one) you can save them a lot of money by not having to have centimeter level accuracy. If your client still wishes to have survey accuracy, then you can develop a partnership with a licensed surveyor that might give you a lower rate if you agree to always use them when needed. Agree to put their company name and contact information on your website under a heading of "partnerships".
Hope this helps give you some good ideas.