How to set up photo sync from iPhone to Mac without syncing Mac to iCloud Photo Library

I am trying to set up my half's iPhone and Mac in a way that the photos are synced to the Mac automatically over the WiFi, at the same time free up space once it becomes full as it's only the 16GB model.
I thought iCloud Photo Library would do the trick but when I enable it on the Mac it wants to upload the entire existing photo library from the Mac first.
I just want it to go one way, iPhone -> iCloud -> Mac, is it possible to set it up this way?
If not possible can I create a second photo library that is blank on the Mac and contains just the iPhone photos? I would rather not but if this is the only way.

Could/should this be handled by iTunes instead? Can it be used, last time I owned an iPhone it handled this but it looks like it's shifted responsibility to iCloud/Photos, is that correct?

Backup is handled by Time Machine on the Mac and the Photo app is used for viewing/managing photos.

Solution 1:

In general settings of your iPhone you can set Optimize iPhone storage and if your iPhone is low on space, full resolution photos and videos are automatically replaced with optimized versions. But all full resolution versions are stored in iCloud.