MacOS – iCloud: Can I control how much space “Optimise Mac Storage” tries to leave on the local machine

After searching some more, I came across the Macworld article “How to exclude your iCloud Photo Library from an iTunes backup”. The article points out that iTunes backups are not supposed to include the iCloud Photo Library data in the first place, as documented by Apple Support:

An iTunes backup doesn't include:

• Photos already stored in the cloud, like My Photo Stream, and iCloud Photo Library

The article reports that others have experienced the same problem where the backup does include the iCloud Photo Library data, and suggests disabling and re-enabling iCloud Photo Library on the iPhone.

I think I will wait for the upload from my MacBook to finish before trying this. Perhaps the problem will resolve itself after the initial upload is complete. I will edit my answer later.

Edit: The above tip may be helpful to others, though unfortunately I have to report it didn’t resolve the issue for me. Apart from disabling and re-enabling iCloud Photo Library on my iPhone, I have also tried deleting its backup in iTunes before making a new backup. But I find that the iTunes backups are still taking up a few gigabytes of additional space with iCloud Photo Library turned on compared to when it’s turned off. I’m not sure what else to try; other tips, or confirmation from others experiencing the same issue would be welcome.

Removing photos from your Camera Roll will not remove them from Shared Albums. It will, however, remove them from iCloud Photo Library, both in iCloud itself and all of your devices.

If you have iCloud Photo Library turned on and the Optimize Phone Storage setting enabled, there's no way to further reduce the amount of storage taken by photos in your camera roll other than simply deleting them (again, subject to the caveat in the first paragraph).

Deleting photos using Photos.app should not affect their storage in the Google Photos cloud. (Deleting them with Google Photos will cause them to be removed from the Google Photos cloud.)