Beeper is owned by Automattic (of Wordpress fame). They’re a corp but I wouldn’t call them mega and they use matrix which is an open federated protocol.

You can run your own matrix server but tbh it’s easier for someone else to do it.

Not to mention the obvious advantages of their bridges into the closed networks of WhatsApp/fb/x/instagram etc

As a bonus, you can run your account on your own server, play with it, and offer to host anyone else's that's interested as your gain confidence