OptiPNG do not know how to choose the correct color mode.
Often for small files OptiPNG selects paletted mode.
This gives a worse outcome than other optimizers using RGB mode.
OptiPNG reduces pallet before compression - this is not the best solution.
OptiPNG should select all MATCHING MODES to the input image ...for more TRIALS.
I propose additional switch:
0 - grayscale
2 - RGB
3 - paletted
4 - grayscale+alpha
6 - RGB+alpha
With this OptiPNG optimizations will be the best on the market ...better in all cases of GIF format.