How to turn off ffmpeg dithering?
https://ffmpeg.org/ffmpeg-scaler.html tells you the sws_dither is the dither algorithm whereas https://ffmpeg.org/ffmpeg-filters.html#paletteuse tells you to turn the dither off.
The answer to your question is to set
paletteuse=dither=none

BTW, http://blog.pkh.me/p/21-high-quality-gif-with-ffmpeg.html gives you the result of the paletteuse settings.