TaragoExp wrote: … How can I delete duplicate photos from the MacBook (to create storage to enable the upload sync) without the risk of deleting the photos on all devices?
iCloud Photos is a synchronization service. When you engage iCloud Photos on a device, then the Library on that device is kept exactly the same as the iCloud Photos Library, and vice versa. So, for instance, as léonie has said, if you take a picture with your iPhone, it is added to the iPhone's Photos Library, copied to iCloud Photos Library, and then copied to the Photos Library on each of the other devices that you have connected. If you delete a picture on your Mac, then that picture is deleted at iCloud and on all the other devices. And, if you delete a picture on iCloud, it will be deleted everywhere else. That's what "synchronization" means.
So, you cannot "delete duplicate photos from the MacBook… without …deleting the photos on all devices."
As léonie suggests, you can keep the same pictures on a device, but take up less room by using "Optimize Storage." This keeps smaller pictures locally and grabs originals from iCloud Photos when they're needed. The major disadvantage to "Optimize" is that you can't back up your pictures, since they aren't always on your Mac.
… I will also move Photos library to external storage drive in the coming days when I get a device that doesn't house my backups.
This solves most storage problems. However, you say you're using a MacBook, and that can be a bit awkward -- you need to be very careful plugging and unplugging the external drive when you move the laptop, since Photos' background processes may be busy. I avoided this by using the external drive for lots of other documents so I could have room for the Photos Library on my internal drive. But that's not always easy. You may need to use the Library on the external drive until you remove excess duplicates.
If you do move your Photos Library to an external drive, be sure to format it in either APFS format or Mac OS Extended (Journaled) format. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:
Move your Photos library to save space on your Mac - Apple Support
