I see it's once again time to post this: https://taggart-tech.com/discord-alternatives
Discord Alternatives, Ranked

Building an online community takes more than tools. But the right tool can make all the difference.

Since I wrote this, many have introduced me to movim, and it's pretty slick! I'm still experimenting, but I like a lot of what I see. Still missing moderation tools for groups, though.

https://movim.eu

Movim – Responsive web-based cross-platform XMPP client

Movim is a kickass distributed blogging and messaging platform built on the industry-standard XMPP protocol

Let me add that I am keenly aware of the cryptographic issues you are about to bring up about any of these options. I read and deeply respect the work of @soatok and others, and understand the concerns around OMEMO for XMPP, Matrix, etc.

Security is a balance between risk and value. I cannot decide for you what the right balance is, but I know from hard-won experience that building a community is about more than "perfect" security.

Some folks are wondering why I didn't include voice chat. It's a fair question! My use of Discord, perhaps as an outlying elder millennial, does not include much voice. But every option in this article except Discourse has voice/video support, and Discourse gains it easily via several plugins, including Jitsi.
@mttaggart As an old genXer grown on IRC, usenet and forums I really hate Discord, but it’s really hard to replace with 1) instant screen sharing/streaming 2) effortless voice chat 3) well integrated meme sharing and custom emoticons which all, and I never thought I’d say this, are my daily drivers for most of my hobby communities.