One of my concerns is that the photos are going to get altered in some way when imported. I've been driven to insanity with an app on my iPad that can only import images from the Photos repository, and .PNG files with a transparent background get converted to having a white background when imported into the photos app. Not that this will be an issue with photographs, but it's clear that Apple does some kind of processing of the raw images.
The import process has, for me, always kept the original image intact. If you use the "copy" option I recommended, the image is stored in the Photos library database as a blob (Binary Large OBject). The meta data is stored separately in another table in the database. Edits you may make (cropping, editing) are stored as individual actions in yet another place. The original is always there, and can be retrieved. If you need the file back, Photos can export to tiff, jpeg, png, and the original format. I don't use RAW format, I use HEIC, and it handles those perfectly. Given that the iPhone takes Live images, it exports both a still frame and the live image as a .mov file, it needs to. The metadata is reattached to the file when exported.
Second issue is that once the pics are in the Photos repository, they can't be edited with a program like GIMP - the repository can't be opened or read by it.
There are add-ons, but I don't use them. Most of the edits I do are simple, and easily done within Photos with the tools there. But it's easy to export an image, when I need to, do more extensive editing using, in my case Affinity Photo, then re-import the new image to the database. The beauty of using Photos for editing is that the original is always there, unchanged, as the database keeps the edits as separate actions. When you open an image, Photos recovers the original from the database, then re-applies all of the edits and presents the image to you as it has been edited. But the original, unchanged, it still there and can be retrieved.
Also, in the edit menu for the image is the option to edit using some external tools. Here is the menu for me:
Note that I can choose to edit in Affinity Photo, in which case the image is transferred, edited, and restored to Photos easily. I don't know if GIMP has an interface or not.
Lastly, one of the frustrations I've had with organizing photos I already have stored in Google Photos is there is no way to tell what albums a particular picture has been added to. I once tried to organize that collection (separate from what I have on my HD) and could never determine which of the thousands of photos I had were "categorized" via an album. I could never determine if I was making progress on getting things organized as I always had this huge pile of photos that couldn't be distinguished from those that I'd already viewed and added to albums.
OK, there are some tools to help. First of all, there is the Library, which is ALL of the images. When you put an image in an album, the actual file doesn't move or change. An entry is made in the database to say that "this image is in this folder" and nothing more. So, an image can be in one or more albums, with little change in the database size because the image isn't duplicated, just a new link created. And there is the function of a "smart" album, in which you can sort out which images are assigned to a given album, or even which images are assigned to NO album, i.e., unassigned.
It's free, play with it a bit to see what it can do. That's the best way to learn about it.
