Actually, no. You cannot read encrypted messages of other users unless you have their key to decrypt it. Encryption/decryption happens on the client side, not on the server, and encrypted messages are stored encrypted on the server.
The ability of admins to read personal messages is an oft discussed issue and won’t be changed. If you are concerned about privacy and security, then strictly limit who has admin access and only log in as admin yourself when you need to change site settings or download a backup.
You seem to be pretty uphappy with Discourse. It is open source so you are welcome to use different software.