P: (AI) Image search for 'similar' in catalog

There are tools that can find visually similar images.  One such plugin is Excire

but your email seems to be more focused on metadata which takes us back to @richardplondon 'a comment.  Some tools can be used to find "similar" for example, if you have an image you can use a date metadata filter and just choose x days before or after the target date.  You than then narrow it down by adding other filters such as camer make, model or serial number.  Similarly you can use a aperture filter and select a range of apertures.  

So, it really comes back to what exactly you are trying to do.

I suppose my two questions are, similar in what way?

(from a similar original technically; with similar metadata assigned; with similar collection memberships in common; with similar processing applied; based on some auto-analysis of the picture content, with similar subject material; or with similar framing / composition) 

And, for the aspects that require some interpretation - similar in whose opinion? (grin)