Installing ffmpeg through homebrew: dyld library not loaded
I can't offer anything to help you w/ Homebrew. I gave it up some time ago due to various issues like this. I can recommend MacPorts as a package manager for macOS.
Once you have MacPorts installed, you should be able to install a functional copy of ffmpeg. I installed ffmpeg on my macOS Catalina system some time ago, and use it regularly without a hitch. Following is the port command I used:
% sudo port install ffmpeg +nonfree

nonfree was one of the &quot;variants&quot; available for ffmpeg; variants are explained here on the MacPorts site