Since Discord has decided to set itself on fire you might be looking for an alternative. Fluxer is open source (AGPL), selfhostable and based in Sweden. But maybe wait a while before you try to install the server yourself because the dev writes on github:

I know it's hard to resist, but please wait a little longer before you dive deep into the current codebase or try to set up self-hosting. I'm aware the current stack isn't very lightweight. In the next update, self-hosting should be straightforward, with a small set of services: Fluxer Server (TypeScript) using SQLite for persistence, Gateway (Erlang), and optionally LiveKit for voice and video.

The developer hosted instance has a Nitro-like premium tier for their service named Plutonium, if you selfhost you get all the premium features for free.

Fluxer is not federated (yet), but according to the website, federation is planned in the near(-ish) future. Furthermore the service is a bit spotty at the moment as they are fighting with an unusually big influx of new users.

Fluxer is an open-source, independent instant messaging and VoIP platform. Built for friends, groups, and communities.

https://github.com/fluxerapp/fluxer

https://blog.fluxer.app/how-i-built-fluxer-a-discord-like-chat-app/

Several years ago I had a Discord community with hundreds of users. This was an IRL community, so it was very difficult to abandon but I did anyway. Tried to get people to leave but they were unwilling. So I handed it off to another member and deleted my account. Now that admin has contacted me again and let me know everyone is ready to leave. I found Fluxer yesterday while poking around #Discord on Mastodon and I think we're going to end up there.

Fluxer is still very early in development and they have plans for many advanced features in the roadmap but it's very feature-rich today. Current monetization plan is freemium + Patreon-like monetization. I understand that may be a dealbreaker for some but there aren't a ton of other great options, and everything is open source, and self-hostable, and if you do, you get all of the premium features for free, while still communicating with the main instance over federation (in roadmap). That still leaves it susceptible to Mattermost-style enshittification but honestly rolling back updates solves most of those style of problems.