Tangentially, as a related example, the free MKV Toolnix suite has a well designed GUI that also provides commadline snippets ( for windows and *nix ) for any jobs you create.

While not as complex as the full FFmpeg CLI options, the MKVToolnix suite has a large option set and having the GUI create usable CLI snippets is very handy for setting up large batch jobs on hundreds and thousands of AV files.

https://mkvtoolnix.download/