Free Anti Virus Software (Replacement for Sophos)

Additionally, the new OS versions are now contained in a read-only volume; that means neither you nor any AV ware is able to make changes or access it.

So, answering your question: you wanted a free solution > it is already on your Mac.

(I'm old school so I remember a time when mac users didn't even have to think about viruses and malware).

Nothing has changed. There are no viruses that affect macOS. There have never been any and there never will be any.

It already protects itself from malware.

The following is the comprehensive list of non-Apple anti-malware products you require:

And that time is now. There are mid-cycle releases of McOS that patch discovered vulnerabilities.

Nobody knows the latest structure of MacOS better than Apple, so any 3rd party that says they can identify all parts of MacOS that do not belong cannot be trusted, my opinion.

Beside keeping your Mac software up to date an occasional scan with Malwarebytes is pretty much all that you need.