Disk Utility Image from "BOOTCAMP" creating empty dmg
Disk Utility's ability to do block-copy was never very good, and was further hamstrung when APFS entered the scene and the job of handling DMGs was handed over to the Finder. It has always had troubles dealing with non-Apple file-systems, which is the main reason Winclone was born.

You're dealing with a container-ised NTFS volume, there aren't any tools in macOS that can do more than send files back &amp; forth. Tim Perffitt's method was to give Winclone its own APFS-and-NTFS-savvy rsync framework, and to the best of my knowledge is unique.

One thought does occur though: You could pop one of Microsoft's free Windows 10 VM images into VMware Fusion or Oracle's VirtualBox (instead of running the Boot Camp install through a VM engine), mount your big NTFS partition into the VM and then use a Windows file-transfer method (e.g. Macrium Reflect or something rsync-based) to shuttle your stuff from the internal partition to an external target.