Congratulations on your efforts so far - not touching the original image file was a brilliant decision.

What does "lsblk -f" show for /dev/sdc ?.
If fsck fails for /dev/sdc1 then the filesystem is probably unusable. fsck rectifies filesystem structure where it can; data within is a byproduct essentially, as you discovered with that pile of file in lost+found.
Testdisk isn't really the tool for user file recovery - especially for modern Linux filesystems; check out its sister product photorec. Does more than just photos, but it scans the disk sector by sector looking for file signatures - it will take forever; maybe a week or more. And you lose all the filenames. The site has a bunch of scripts to help there.

Thank you for the "compliment", unfortunately I am not a stranger to data recovery. Between brown outs and cable trips as small examples I have seen data failure a few times. I did have a backup but that is a greek tragedy for another time. Usually I can pull something and I am not left holding an empty hat per se but this time I am banging my head against a wall.

Thank you for the help, everyone....One step at a time.

****

When I ran lsblk -f it provided me with this output:

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administrator@OPENMEDIAVAULT:~$ lsblk -f /dev/sdc
NAME FSTYPE FSVER LABEL UUID FSAVAIL FSUSE% MOUNTPOINT
sdc ext4 1.0 DATA f4e8bb36-ca09-407b-98d6-7a621277cc7a
administrator@OPENMEDIAVAULT:~$
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This is a brand new dd-extract with nothing done to it. Freshly written.

lsblk -l provides this output:

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administrator@OPENMEDIAVAULT:~$ lsblk -l /dev/sdc
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdc 8:32 0 3.6T 0 disk
administrator@OPENMEDIAVAULT:~$
---------