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

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.

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.

I'd try matrix apps like element or wait for spacebar (formerly fosscord), no much alternatives available unfortunately.

Matrix

Been checking out open-source alternative Revolt, which is very similar to Discord but currently very bare bones, biggest feature missing is screen share.

Possibly Rocketchat

Linen (https://linen.dev) can sync Discord chat, it doesn't have voice or video capabilities though but at least backs up the important content.

Matrix is probably the best solution. There is a bridge feature that allows you to merge a Matrix room with a Discord channel to the members can tack to each other. This allows for a smooth transition of communities away from Discord.

In case you guys are wondering, despite being posted to [email protected], this post actually only exists and can be interacted with on fedia.io

Federation seems to be falling over all over the threadiverse.