How to clear out this Photos/ Camera Roll related data taking space on the iPhone even after Deleting all Pictures?
It is likely that a wipe and restore will fix the issue.

As the iPhone is essentially a pocket computer with a simplified and locked down interface it is difficult, at best, to get into the inner workings to apply fixes that are needed to solve a particular problem .

I have found that many issues like this are readily solved by a simple, though tedious, process.


Back up your iPhone via iTunes to your local computer. Give the backup a password so it saves the app passwords already on the iPhone
Go to Settings > General > Reset > Erase all content and settings and wipe the phone
When the phone reboots, plug it back into the computer and restore from the backup you just created.
You will have to now enter AppleID/iCloud passwords to reconnect it to your Apple accounts


You should now see that the space formerly occupied by (deleted) photos and the like is now freed up.