see: https://thereallo.dev/blog/converting-gifs-properly
tldr:
; ffmpeg -i input.mp4 -vf "fps=15,scale=720:-1:flags=lanczos,palettegen" palette.png
; ffmpeg -i input.mp4 -i palette.png -filter_complex "fps=15,scale=720:-1:flags=lanczos[x];[x][1:v]paletteuse" output.gif