I use to be in the same position, when I attempt to render, my Miencraft would crash.
However I was able to get a solution from the advice I got in this thread.
https://www.replaymod.com/forum/thread/529
TR;DL - Instead of installing FFmpeg through terminal (eligible for mac users only), I directly downloaded it and placed it on the folder path /usr/local/bin to get my ffmpeg. If you have a Windows and you are having this issue, this thread may not be helpful.
Yeah, sometimes Homebrew doesn't do everything correctly.
If you need help putting the ffmpeg executable in /usr/local/bin, feel free to reply.