Many years ago I used XMPP with Prosody and it was ok.
- You can setup some retention time for attachments and history depending on the space you have.
- There are clients for multiple platforms but not very much advanced vs competition.
- Security is good with omemo protocol. In so e clients you have to turn it on since it's not by default.
- Audio video calls on one to one is good as long as you setup stun coturn server. Android to iOS calls still in progress.
- Check out Snikket for an all in one solution.
You might want to also consider Matrix (dendrite option ?) for your needs.