Didder is fantastic. I'm using it to convert images to grayscale for use with Waveshare e-ink displays. I've found no other tool that both lets you specify a custom palette (without which the resulting image looks pretty bad) and is reasonably quick.

For future reference, this works beautifully with their 9.7" display: didder --palette '000000 111111 222222 333333 444444 555555 666666 777777 888888 999999 aaaaaa bbbbbb cccccc dddddd eeeeee ffffff' -g -i - -o - edm --serpentine FloydSteinberg