Don't use gradfun2db. And not with that strong of a value. You're killing detail.
Get the dither package
and use gradfun3. Follow the manual to learn how to tweak the parameters. Also, do use it only in the scenes that do need the debanding, with a filter like strange
. Another thing you can do to help is adding static grain (not much, just a tidbit), which helps hiding part of the banding, makes the picture look more detailed (keyword being look: it's not more detailed, just feels so while watching) and helps out preserving gradients in the final encode stage with 8bit encoding.