I tried installing Imagemagick using 'brew install imagemagick'. The end result I was hoping for was the existence of commands mogrify and convert. It had apparently been a long time since I ran Homebrew; tt thought for a long time and then went about updating a positively frightening number of packages. It ran for perhaps 45 minutes, and I really have no idea how much it updated. Dozens, scores, perhaps hundreds of packages. Including Ruby and Python, which was pretty scary in its own right.
After all that time Homebrew reported failure because something it was trying to instlal required a later version of Xcode than was installed. It turns out that the version it wanted was part of Catalina, and my OS is still at Mojave, so I can't upgrade Xcode to satisfy Homebrew.
When all was done, the mogrify command exists and works but the convert command is nowhere to be found.
I have tried redoing 'brew install imagemagick' -- it says the package is already installed and refuses to do anything. 'brew upgrade imagemagick' worked once but there is still no convert command to be found.
Do I have to uninstall and start over?