May be a little extreme but it seems like you'd know what you'd be doing: have you tried deleting the Photos library file (found in the Pictures folder in Finder) from your iMac? You would have to turn off iCloud Photo Library or you could jut reassign iCloud Photo Library after the deleting the old Photos file and creating a new library (after you relaunch Photos and it says Library not found).

I did this once before prior to transitioning from 10.11 to 10.12 because I needed the SSD space, but 10.12's in the cloud features helped me regain a lot of storage so I recreated the library as a new one. I remember running into trouble and getting "iCloud photo library can only be assigned to the main library" (or something) errors but it was easily fixed (although I don't remember how, so be forewarned).

I'm only suggesting this since you say iCloud Photo Library is fine on the iPhone and, since the Library is now on iCloud and not stored locally (on the Mac), you really have nothing to lose.