Posting this here for posterity since I had a lot of issues finding a way to successfully convert images, this is based on the posts by mergen3107 and NiLuJe (thanks both!). Using ImageMagick and the gradient file NiLuJe posted, the below powershell command will convert images with no issue for an Oasis (change resolution, etc. to match your model):
convert input.png -filter LanczosSharp -resize 1264x1680 -background black -gravity center -extent '1264x1680!' -colorspace Gray -dither Riemersma -remap eink_cmap.gif -quality 75 -define png:color-type=0 -define png:bit-depth=8 png
ut.png