I have tested a few of these for a small group (~15 people). My experience:

Matrix (Element) — The most feature-complete option. Spaces work well as Discord server equivalents now. The big downside is Synapse’s memory usage (easily 500MB+ with a few rooms). Dendrite is the lighter alternative but still not fully stable for federation. If you are keeping it private (no federation), Dendrite works great and uses way less resources.

Revolt — Closest to Discord in UX. Self-hosting is doable but involves several microservices. Good if your users expect a Discord-like experience and you have the infrastructure.

Mumble for voice — If voice chat is important, do not sleep on Mumble. It is ancient but the audio quality and latency are excellent, it uses almost no resources, and the setup is trivial. We run it alongside Matrix for text.

The age verification situation is pushing a lot of communities to finally make this switch, which is honestly overdue. Self-hosted communication should be the default for any group that values its privacy.