I think that severe gouging is why you got so much removal with such a fine polish. It takes less to cut down a thin ridge than a flat surface. The spaces between the gouges help hold fresh abrasive.
Back when cars were easier to work on and more people did their own wrenching, there was a tool that looked like an old fashioned egg-beater for valve lapping. The gear teeth on the crank plate alternately engaged gears on the lapping shaft above and below its axle. That's how it kept reversing the direction of the shaft with each rotation of the crank. That would be the perfect tool to attach to your case laps.
This appears to be one, though the description is limited.
Do you have a comparator you can put on the headspace gauge and compare to your fired cases? The caliper adapter types, like the Hornady LNL or the Sinclair tools would be fine. So is the RCBS Precision Mic. None of these produce very precise absolute numbers, but for comparison to a gauge as a standard, they are just fine.
If you still have more room to remove material, you might consider slotting the fired case and using coarse then fine silicone carbide valve lapping compound to do your coarse cutting before polishing. You'll save a lot of time.