To address both sand and the Harkonnen scene, a hybrid libvpx profile is proposed:

ffmpeg -i "Dune_Part_Two_Source.mkv" \ -c:v libvpx-vp9 \ -crf 18 \ -b:v 0 \ -cpu-used 1 \ -row-mt 1 \ -tile-columns 2 \ -thread-count 8 \ -pass 1 -an -f null /dev/null && \ ffmpeg -i "Dune_Part_Two_Source.mkv" \ -c:v libvpx-vp9 \ -crf 18 \ -b:v 0 \ -cpu-used 1 \ -row-mt 1 \ -tile-columns 2 \ -thread-count 8 \ -pass 2 \ -c:a libopus -b:a 192k \ "Dune_Part_Two_Encode.webm"