Yes, those all work and each require that you download and install their app, go through setup, potentially some identity verification steps, etc.
If you want that functionality, all of them are available as options.
What would make an Apple iMessage app for Android better than any of them? Unlike today, Android users would have the same experience for any of these other apps - completely excluded from conversations until everyone agrees upon an app, downloads it, creates an account and exchanges whatever addresses, nicknames or QR codes necessary to join a group.
The only thing that an Apple iMessage app buys the group is a better experience for the _Apple_ users. It actually increases lock-in to Apple's services, both because now Android users are signing up for Apple services to to communicate with their groups, and because Apple users know they can just reject other options because the Android people can "make iMessage work".