The FAQs I have read for using iCloud Photo don't explain how/if I can delete my iPhoto library after all my photo's have uploaded to iCloud.
How did you upload to iCloud Photo Library?
iPhoto cannot use iCloud Photo Library, only the new Photos for Mac app can do that. Do you have an iPhoto Library and a Photo Library? Please explain.
The usual way to save space on Mac by using iCloud Photo Library is to enable the Preference setting "Photos > Preferences > iCloud > Optimize Mac Storage". Then the originals will be stored in iCloud and only smaller, optimized previews on your Mac. But you will be able to browse your photos, if you are offline.
If you have both an iPhoto Library and a Photos library, the combined libraries are sharing the original image files and do not need much additional space, see: Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support
So deleting either of these libraries will not free much space.