Best way to import (only) iPhone images into Photos.app from many folders?

For instance, should I simply go through the folders on my hard drives and copy images shot on iPhones into a new folder and then import that folder into Photos – is it that simple?

Yes. Or even more simple, import them from where they currently reside. Photos copies the images into the DB, it doesn't move them or affect the ones in your folders in any way.

Or would it be better to first copy all iPhone photos into a similar folder structure (by year, event or such) and then import each such folder?

Nothing to be gained. Photos displays things by date and time automatically.

It might be worth importing a few folders as a trial to see how it works. Bridge is essentially a file browser. Photos (a bit like Lightroom) is a database that does a lot of things automatically. It's designed for folks who don't want to spend a lot of time curating their images.

Smart albums are albums you can create that select photos out of you library based on conditions (usually in the metadata)

Here is the smart album on mine to do what you would need (Selects nearly 7000 images out of my nearly 10500 total.:

But I agree that importing 3TB of images is not a good idea 🙃

I've also just found a spotlight search you can use to identify all your iphone images. You should be able to just drag from this search into photos:

The easiest way might be to import all your photos in a first step, then use smart albums in photos to identify automatically which ones are NOT iPhone picutures, and delete those from the library.

EDIT - the spotlight search finds files on your hard drive - not in photos, so can be used to select files to import. You can limit the search to a particular subfolder tree if you wish.