Here's another thought in favor of the mill. If you've not
already done so, switch to the water soluble polishing compounds.
Clean-up is SOOOOooo much easier and would make doing without the
ultrasonic less problematic!
But Judy, cleanup of standard compounds isn't that tricky either.
Just do it the way it was done before ultrasonics. Even with them
around, some shops still just use the old boil-out pots, since they
don't damage as many things as an ultrasonic. An old saucepan on a
hot plate, filled with a moderately strong solution of lye, drain
cleaner, or any other strongly alkaline cleaner (even TSP does a lot,
though it's not QUITE as effective on stubborn caked on compounds)
Use it with ventilation, and let it heat to a simmer just barely
below actually boiling. Work soaked in that for a few minutes will
not have polish compounds on them either. Save for all but a few
mostly organic stones, or those with oils or treatments that would
come out, but they can't go in the ultrasonic either.
Ultrasonics are a convenience, saving you time. But they really
don't do anything you can't manage just as well, without one.
Rolling mills, on the other hand, are an almost essential tool.