Larry, if you want to end up with large flat surfaces, your best bet
is to use a cast iron lap with loose silicon carbide grit. This is the
fastest way. Using the drum wheels you are talking about will result
in dips and indentations that are not obvious until the next stage
fails to remove them. If you have concave surfaces to work, the tool
will have to be smaller in diameter than the radius of the concave
part. There is nothing wrong with your combination of wheel grits,
just the wheels.
The least effort required would be to use a vibrating lap to get the
flats down. It takes a long time to do but you don’t have to be with
it every minute.
For marking the material to see where you have been and where you
need to go, I use a sharpie marker. cover the entire surface you
are working then use a fine sand paper, 600 or so, on a hard
board and do a quick light sand of the surface. This will
take off all of color from the high spots and leave the pits,
scratches, holes, etc. that you will need to work out.
One more point, watch the heat buildup, especially during the polish
stage as you can easily pop the quartz. You have a lot of thermal
mass to work and it is very easy to get the surface hot and cause