How to keep some photos separate from main library?

In recent years I have used an iphone for photos with the library consolidated on a mac.

However I also have a large assortment of older photos and old scans from other sources that I would like to use the mac tools to organize and de-dup without merging with my main library, at least until I can remove unwanted ones and fix the timestamps on the ones I do want to keep.

What is the best approach for something like this? Add a dummy user on the mac to have a separate library? Change the library location in photos and turn icloud sync on and off when switching? Something else? Is there a handy way to set 'approximate' timestamps on a folder full of photos or scans before or during an import? If I do end up with a set that I want in my main/icloud library, is there a direct way to do it or would I have to export and re-import?