https://fluxer.app seems to be the best alternative so far, by a mile. Gained a bunch of users during some controversy too.

Someone else here also mentioned Chatto [0] which seems far easier to self-host than Fluxer, though both appear to be quite similar to Discord in overall functionality. I've played with Chatto a bit and it seems surprisingly solid overall thus far.

[0]: https://github.com/chattocorp/chatto