To get rid of Viruses, simply clean out all executable attachments in mails, mailcow and other solutions support that.
You can also mount /home nonexecutable, which means everything you can run needs to be on the system. Without that, “control over what is installed” is worthless. You could literally download any package, export the binary and run it from anywhere.
To run untrusted software, you can use a server that uses something like KASM. It is image-based, accessed through the browser, suppports uploading files and viewing lots of stuff. You can also run antivirus there, but as shown in this video antivirus is often simply tricked by encoding and re-encoding the scripts into something like Base64.
Antivirus really is flawed. You need to control the origins of code, and run all untrusted code in immutable VMs.