On the Apple Support Communities, I found out that “phantom photos” were taking up the space, but how would I delete them and reclaim the free space? After much searching, I found out that by manually changing the date back 3 months, 6 months, perhaps as much as one year (Settings > General > Date & Time), old photos would be revealed in the Deleted folder that really never got deleted. Once they were visible in the Deleted folder, I deleted them all.
Now the Photos app only takes up 94 MB, probably just enough to let it run and all those GBs of space are truly freed up. Don’t forget to reset your Date & Time back to “Set Automatically” so that your iPhone is back in the present time.