Install cask that extends the brew command :
brew install phinze/cask/brew-cask
Install calibre using cask :
brew cask install calibre
Setup $PATH for calibre (add ebook-convert and other commands to $PATH) :
export PATH="~/Applications/calibre.app/Contents/MacOS/:$PATH"
Or add it to your shell's .profile with :
echo 'export PATH=~/Applications/calibre.app/Contents/MacOS/:$PATH' >> ~/.profile
Now ebook-convert should be available :)
for M1 apple silicon
works