FYI for future readers, while iMazing backups can be located on any drive, iMazing requires and uses the ordinary iPhone backup files that are in the default Mac location (~/Library/Application Support/MobileSync/Backup), so you don't actually free up any space on your main drive by using iMazing.
It may be possible to create a symlink at ~/Library/Application Support/MobileSync/Backup that points to your external drive so iPhone backups are written to the external drive instead of your internal drive. But I have not yet tried that method. It may or may not work. (For me, using a symlink worked for Mac Mail but not for my Outlook profile, so I have not tried it for iPhone backups.) Note also, in my experience, symlinks have become progressively more difficult to do successfully with each release of macOS since High Sierra.