How do I delete iCloud Photos from iPhone

I have an old iPhone 6S and have been using iCloud Photos to sync all photos from iCloud & my Mac, however, my library is so big it has run the 6S of storage (even with optimize Photos turned on). I've decided to revert to importing photos to my Mac manually every once in a while until I eventually upgrade the phone.

In an attempt to recapture some storage on the iPhone, I turned off Photos in iCloud and deselected iCloud Photos in Photos settings. I received the warning that all photos would be "removed from your device but remain in iCloud" but the optimized photos (26,000) have not gone away on the phone. I can go into Photos on the phone, select and delete but 26,000 one at a time? Right...

How do I get these optimized iCloud Photos to go away on the iPhone to recapture storage/space??