We start a new app. Opensource Discord, Self-hosted, federated. Serving that subsection that cares about privacy and security.
Discord is a good design, and should be replicated rapidly with mutations from competitors galore.
> Opensource Discord, Self-hosted, federated
Sounds like you want https://matrix.org/
> Discord is a good design
Then the main, reference client https://element.io/ or https://fluffy.chat would work great for you.
... With the only caveat being that general experience of using Matrix is awful.
I second the other commenter's suggestion of using https://stoat.chat/ or as it used to be called: Revolt, which matches the "Opensource Discord" requirement perfectly.
Mumble already exists. IRC exists. Matrix exists. Discord is a surveillance tool by design. Jason Citron pulled the same hijinx with Aurora Feint, but I assume he has been betraying users to CIA-and-Friends from the start so he gets a pass for breaking the same laws.
Nobody scales free, high-bandwidth services without some dark money support from feds or worse.
Revolt/stoat has existed for quite a while: https://itsfoss.com/revolt/