> ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 30 -b:v 2000k output.webm
https://explainshell.com/explain?cmd=+ffmpeg+-i+input.mp4+-c...
It doesn't seem to know what to do with combined flags in this case.
I found a modified a ffmpeg shell script no later than yesterday that finds all my movies files and generates one jpeg with 16 "tiles" thumbnails for each movie to try to make sense of the countless family "MOV40931.MOV" and whatnots and I've got to say: I agree with you.
My big moment of solitude was when I started to work on parallelizing my shell script only to realize that, of course, ffmpeg already makes good use of all the cores so there's no point in parallelizing the shell script.
I wrote a script called dovid I left on Pastebin that creates contact sheets (what you are describing) for every video file of any popular format in the directory executed and then combines them into a webpage or pdf. But it isn't an exceptional script because it just utilizes vcs[1] and convert (ImageMagick). At some point, I stopped using the script and just aliased this command (arbitrarily as 'peak') in my ~/.bash_profile
alias peak='for i in *.mkv *.mpg *.wmv *.avi *.mov *.m4v *.mp4; do echo "$i"; /opt/local/bin/vcs -q -q -j -dt -ds -dp -u "0" -n 16 -c 4 -H 162 "$i"; done; echo ".......... converting jpgs to a pdf"; /opt/local/bin/convert *.jpg index.pdf; /bin/rm *.jpg'
[1] http://p.outlyer.net/vcs (ffmpeg is a dependency)