Mumble would be my bigger recommendation for a truly open source Discord alternative, though I'm personally more invested in XMPP as an alternative.

Mumble is fantastic for voice chat. Its text features are very basic, though, so people fleeing Discord would probably want something additional to handle that. Maybe Matrix.

A single location is a good selling point. Being able to jump into a voice chat, and still post things in a shared text chat is a good feature. Mumble should work a bit on that.

> Mumble should work a bit on that.

Mumble is a labor of love, not a commercial product. I expect they would appreciate your help.