Cannot delete photos from iPhone. Trash can greyed out.

I've had a problem of photos and albums not syncing to my iPhone (6 plus) from my MBP (Retina, 15-inch, Mid 2015). So, I decided to remove all photos from the iPhone and 'start over'.

The problem is that there are 6k photos on my phone that I cannot delete (trash can greyed out) and they cannot be 'seen' by Image Capture or Photos Mac. Here is what happens:

First, sync iTunes with Sync Photos selected but 0 photos selected --> 0 photos shown for iPhone summary

Then, sync iTunes with Sync Photos Deselected -> “The iPhone could not be synced because the sync session failed to finish.” And there are 5GB of photos shown on the phone for summary

So, iTunes 'sees' 5GB of photos when 'Sync Photos' is Deselected, but does not 'see' the photos when 'Sync Photos' is selected but zero albums are selected to sync.

Image Captures shows no photos on the iPhone. I used Image Capture to remove 2+ GB of photos but it does not see the remaining ones I'm trying to remove.

Photos (macOS) sees no photos on the iPhone. Photos iOS DOES see the pics and they take up 5GB of space.

No photo on the iPhone can be deleted: Trash can is greyed out for either one photo selected or for multiple ones selected.

Also, I have never used iCloud Photo Library. Only used iTunes sync. My Photo Stream and iCloud Photo Sharing are turned off and have been for a while.

I have thrown away iPod Photo Cache already. Restarted several times. Running the iOS 11.3.1 and macOS 10.13.4.

Soo, any ideas? How do I remove all photos from the iPhone? And how to reset the sync with iTunes?

Seems like the photos are clearly on the phone but are not accessible to remove or anything.

Any help is very appreciated!