Implementing MDM with Apple DEP

I’m working on implementing Mobile Device Management (MDM) with Apple's Device Enrollment Program (DEP) on my company-owned iPhones.

The implementation steps I’m attempting to take are:

1. Backup iPhone using iTunes over a USB cable.
2. Do a factory-reset on the iPhone, so the device will become "supervised".
3. Restore the backup from iTunes, so the user’s photos, text messages, apps, and custom settings are not lost.

Unfortunately, after the backup is restored, the iPhone is no longer supervised.

If I factory-reset the iPhone, without restoring the backup, it remains supervised.

This is a BIG problem.

My users will revolt if I tell them they have to do a factory-reset and start-over from scratch with a blank iPhone.

I cannot be the first person to implement MDM on a group of existing iPhones.

How do I get my iPhones to be supervised while retaining the user’s settings and data? Please help!