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.
It depends. Are you doing this for an organization? If so, mattermost is a very good option if you want good, concrete division between topics and a slack-based UI. If you want 1:1 or small group communication between people who know each other, Signal is the gold standard. If you really need privacy or are doing 1:1 communication with people you don't know, use Briar. If you want cost over all else, matrix is very resource intensive and XMPP is a better choice. What do you want to do and who do you want to talk to?