Matrix? It's also quite close and federated as well.

Since you mention you have Nextcloud, it's worth knowing that Nextcloud embeds Spreed which gives you chat, audio, video, screensharing and a ton of other features. The trick with stuff like that is getting people to sign up, but limiting it to only people you want to chat with. The one thing that's valuable about internet-facing stuff like Discord is that anyone can sign up and it's not your problem. With Nextcloud, you can often federate and let people sign in with Google or Github or Facebook or whatever, but—as an individual—you probably don't want to have truly open signups on a personal chat server. So you'll have to sorta invite/accept people signing up. Matrix (already mentioned) is the other good contender.

Matrix

There's Revolt that seems to be very similar to Discord. They even have instructions to self host on their github repo. Don't know if this matters, but as of right now it is not federated.