Syncing Photos.app with external HDD I want to have my photo library to sync all the pictures on the HDD, which I can easily reference them by deselecting "copy items to the photo library" and import them.

but I want that if I delete the photo in the photos.app, then it would delete the referenced file too. Is that possible?
Importing without coping to the library doesn't give Photos.app ownership of the file. So to be safe you can't delete it directly from the application. You can right-click on the picture and select Show related file in Finder and delete it from there. Unfortunately, it will not delete it from the photo library so you have to do it by yourself.