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.
Matrix (Element is just one compatible client) is up there if Signal is too limiting feature-wise or too sus for yah. It can be annoying but not terribly hard to self-host, there are multiple server and client implementations that generally interoperate (though currently the server side you should probably stick to synapse unless you're resource constrained or know what you're doing)
if you don't like the UI of element, try fluffychat or nheko or something
Just take note that Matrix was originally developped by Amdocs, an israeli multinational with ties to the state intelligence. They made a foundation to pursue further development, but apparently the staff is the same.
It might not mean much, but many people don't know about it.
Matrix also also really, really bad when it comes to metadata. I would not trust Matrix when it comes to sensitive things or communication that can be used against you due to how much metadata it leaks. I also find the double standard here were people don't trust Signal due to funding but ignore funding issues in Matrix.
I haven't used that feature but a bunch of non-element clients seem to support that according to this: https://matrix.org/ecosystem/clients/ (filter by feature)
Of them, I would say Hydrogen web, and FluffyChat, are the most serious based on features supported and what I've heard about them. I haven't used fractal since before it supported e2ee though
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?
Matrix is probably the best but keep in mind that all messaging apps can have their metadata collected somehow and here's an fyi for matrix itself: https://hackea.org/notas/matrix.html
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.