Ok, so this centipede printer workflow isn’t really reliable so I dove into CUPS to try and see if there’s a way to alter the dithering.
I tried adding different settings to the PPD file but nothing produced any changes.
Until I added “DitherPPI” (a GS option) and it produced a result that was pretty good! I tried various values (30, 80, 120, 180) but they all looked identical. In any case, this is good enough for me!