Matrix?

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

Matrix.

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

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

Dark City, the superior version of the Matrix.

Revolt exists, idk much about it

revolt.chat ?