It sounds like you are spending way too much time per wet. With the 80 grit do about 20 strokes and clean off the mirror and see what the appearance of the surface is. The edge should be a lot more like the center of mirror. Also make sure that you apply enough grit (but not too much!) so that the grit covers the whole surface of the tool while grinding. I might also note that you aren't going to get too long of a wet with that coarse grit as it breaks down (as well as doing a lot of glass removal) rather quickly compared to the finer grits.
When grinding, note the sound that the coarse grit makes when starting the wet and when that sound gets a fair bit quieter, it's time to stop and clean up the tool and mirror - you don't need superclean but rather just a quick swish to get the big stuff off.
Since the purpose here is to dig a hole, I recover the grit after a session and put it back into the bottle (I use the grits wet with about 1 part grit and 4 parts water in one of those Evian poptop drinking bottles - 1/2L size is a good size). Don't try to get all of the grit but just the stuff that falls fast to the bottom of the bucket. That's the 80 grit while the other stuff is more like 200 grit or smaller.
The 1/3 stroke (about 2 1/2" on the 8") should keep the ROC about where it started although it will slowly decrease the ROC. If you are moving fast then you want to decrease the stroke length a bit but you have to remember that, providing you don't force the wear pattern, whichever is on top should be turning concave. There is a natural stopping point tho on the ROC as you go along.