I'd been thinking Telegram would be a sufficiently secure alternative, but as Western intelligence gets their hooks into that system I think we need to go self-hosted. Element is the biggest name here, but I'm curious what options are the best for a combination of both security and feature maturity.
Sorry, I don't know much more than that. The group chat I mod for is transitioning to matrix from discord. We'll probably checkout xmpp once we are mostly done, but we don't want to alienate users so we use repost bots and its going slowly.