Quicker way to copy files to a set of different folders for classification purposes I think this is a pretty classic setup, hence I'm hoping for a solution here.

In the finder, I have a folder "pictures" containing 15,000 pictures.

For each of these pictures, I'd like to send it to a sub-folder of the folder "categories". For instance, if there is a picture of a boat, I want to copy (not move) this picture to "categories/boat".

Currently I have two finder windows open, one with "pictures" and one with "categories" and I drag&amp;drop every single picture one by one, while holding the Option key (to copy instead of moving).

Doing this with a mouse or trackpad becomes tedious very quickly and is slow and error-prone. I am open to any suggestions, even indirect ones leading to an identical outcome.

The absolute ideal would be that I could assign keys 0..9 to each destination folder, then I select a picture in the Finder, hit "5" and it goes to the destination folder assigned to 5. But I must be dreaming, right?
For each of these pictures, I'd like to send it to a sub-folder of the folder "categories". For instance, if there is a picture of a boat, I want to copy (not move) this picture to "categories/boat".


I don't recommend this approach.

This just creates multiple, on multiple, on multiple duplicates of the same file.  Just doing a little bit of math, if each photo applied to just 3 categories, storing them in your method, the number of photos you would have goes up to 45,000.  

What you need is Photo (or image) management software which is just a very specific content management system.  The way this software works is by embedding meta data into the photo (or in Lightroom's case, they create a library of meta data that gets associated with the file, never touching the file directly).  So, if you had a photo that fell into 5 different categories, you wouldn't have 5 identical copies of the photo, you'd have one with 5 different ways of finding it.  Several of these applications have "dupe finders" which is a feature designed to solve the problem of the same file in multiple places!

Here's a list of some popular Photo Management products.  I'm very familiar with Bridge and Lightroom as well as ACDSee from the 1990s and I recommend them wholeheartedly.  I have had some brief experience with the Open Source products (FOSS) and they look to be very promising but depending on your needs, YMMV.


Apple Photos (Included with macOS)
Adobe Lightroom
Adobe Bridge (Free with purchase of Photoshop or Illustrator)
Cyberlink PhotoDirector
Magix Photo Manager
ACDSee Photo Studio 
Darktable (FOSS)
LightZone (FOSS)