with that motor i,m getting 2800rpm at 12v (bit of drag on the angular contact bearings)

I wouldn't get you knickers in a twist about voltages... use a 12v power supply and let the ESC take care of the speeds (just dial it in with the servo tester)
I think between 1500 and 2000rpm would be a better speed with this motor so if you could squeeze a 4:1 you may be getting closer to where all the grunt is

a problem you may have with this route is at startup the toque is pretty low, if the grinder is loaded up it may struggle to start (im not up to speed with coffee grinding procedure, I assume you would run it dry and then knock it off?)