how do I delete all the photos from my phone, once they're uploaded

Did the Mac Photo app not offered to delete the photos after you've imported the photos onto you Mac?

If you have all the photos on your Mac, you can delete the photos on your iPhone.

1. In an album, tap a photo or video to view it in full screen.
2. Tap .
3. Remove the item from just the album or delete it from iCloud Photos on all your devices.

If you have lots of photos then it is easier to import to you Mac again and use Mac Photo app delete for you.

Delete photos on your iPhone, iPad, and iPod touch