Unidirectional sync from iOS device to iCloud Photo Library
Using Photo Stream does mostly work.

Photo Stream images from the phone do end up in my iCloud Photos Library. I assume that either my main iPhone or Mac grabs the photos and adds them.

If the devices did not sync for > 30 days, those photos probably would not be synced to iCloud. But that's not a huge limitation.

The only major annoyance is that Photo Stream does not sync Live Photos. Only the stills end up in iCloud.