I have an older iPhone with limited storage. I use it as a "distraction free" version of my main phone for the gym. I've deleted almost all apps except for Music and Podcasts.
I want to be able to take photos with this iPhone's camera, and I want them to be backed up and uploaded to my other devices / iCloud Photo Library.
I do not need to view other devices' photos on this device.
I believe this may be a somewhat common intended workflow for people with multiple iOS devices. Especially older iPhones, iPads, and iPad Touches being used for specific purposes.
I would prefer to use "Photo Stream" so that no iCloud photos are downloaded to this device:
Upload to My Photo Stream
Upload your last 30 days of new photos and view them on your other devices using My Photo Stream. Photos from other devices can be viewed in the My Photo Stream album, but are not automatically saved to your library.
But it's unclear if the photos are ever automatically saved into the iCloud Photo Library.
Photo Stream also does not sync Live Photos or Videos, which is not ideal.
Using "iCloud Photos Library" would be fine, but Photos has already downloaded 16.49 GB of photos and caused an iPhone Storage Full alert, despite having "Optimize iPhone Storage" turned on.
It does not appear to be possible to strictly limit the size of an iOS Photos Library:
Is there a way to enable unidirectional iCloud Photo Library?
Photos should be uploaded from this device, but ideally should never be downloaded to this device from iCloud / other devices.