There is no switch in Safari. As the blog post mentions, you'd have to use a Safari extension such as StopTheMadness Pro.

A generic way to stop this across all devices would be great. I don't suppose something can be done with DNS via Pi-hole?

You could block account.google.com (or google.com) if you don’t use any google services or expect anyone on your network to use any google services*.

* I think search still works without an account, but I don’t think anything else works that way these days. The last I know was YouTube and I get a “sign in to confirm your age” on YouTube on most videos I try to watch without being logged in.

As much as I'd love to do that, it might be a bridge too far...

This option never worked for me, I also tried the options under myaccount.google.com with no luck, I kept getting the popups and ended up having to switch to Brave and Safari to get rid of them.