I have tried everything (I guess), such as writing my sasl_passwd and editing main.cf in the
/etc/postfix folder (as described in 1). I also copied my LaunchDaemon plist with a different name to my
/Library/LaunchDaemons folder (as described in 2).
While those people managed to get postfix to send smtp Gmail messages from their terminal under Mojave and El Capitan, respectively. I haven't managed to get their solutions to work under Catalina.