I run an XMPP server at chat.thraeryn.red.
For those who don't know, XMPP is pretty simple, text-based messaging. It has always been federated, with users on different servers adding and messaging each other freely. (It's what GMail/Google Chat used to use – in the typical corporate fashion, to lure everyone in before closing and locking the doors on their users.)
My server has end-to-end encryption turned on by default; it has the ability to send and receive (reasonably sized) files; it has the ability to fire up a group chat; it can talk to users with XMPP accounts on other servers, no problem. Encrypted files stored locally are auto-deleted after four days.
Mobile and desktop clients are available for almost all OSes and platforms. Keeping things synchronized across multiple devices can be a little odd. My personal choice? I have a client on my phone that I use the same way I'd use another secure messenger like Signal or WhatsApp. (Except, y'know, a company doesn't run this. Your buddy RED runs this.)
You're officially invited to create an account, if you'd like. I'll get a notification that you have, and I may say "hi" because I like people; e2ee and regular storage deletion ensure those are about the only interactions I can have with you, your chats, or your data. (I will have the ability to delete your account if I receive reports of abuse, and I will exercise that ability.)
The server is
chat.thraeryn.red
and the XMPP Standards Foundation maintains a list of various clients here.