Reminder that BlueBubbles and AirMessage both are working and fairly robust. I've used them daily over a year. The downside being that they need a Mac and iPhone to run. But in the spirit of self hosting, you do run the server yourself and don't share your credentials. I don't see a more viable path in the near future.
Then why would anyone use BlueBubbles? If you already need the hardware, and presumably an Apple account, what advage would there be? Legitimately curious.