Google Photos deletes copy and original on device
After a while of trying around, I found a solution.
This behavior of the Google Photos app seems to be intended, because it is more an interface for the network service to synchronize images, and less of a gallery app. It keeps track of exact copies and synchronizes them as one. Therefore they are deleted as one instance, even though they are stored multiple times on the device.
Nevertheless, there is a (rather hidden) function implemented that does exactly what I wanted instead.
Instead of using the trash icon...
                                         
                            Screenshot of a device folder opened with Google Photos. Trash icon highlighted.
... under the available options, when an image is selected, there is a &quot;Delete from device&quot; option doing the trick:
                                         
        Screenshot of a device folder opened with Google Photos. Option menu open. &quot;Delete from device&quot; highlighted.
Note: With this method the image remains in the cloud even if it's removed from all synced folders.
I hope I could help someone who has encountered the same problem.