MacOS single user mode: how disckutil secureErase main drive?

Hi

I have an iMac mid 2011 with faulty AMD 6970M 2 GB videocard. It cannot start up properly, the boot-up stops right after the white display with grey Apple logo and then the iMac starts looping. In 2014 the AMD videocard was replaced during the Apple official AMD 6970M replacement program.
Unfortunately even the replacement unit was faulty after 3 years.
My local Apple Store refused to apply another replacement because the replacement program is terminated.

I would like to wipe out the entire hard drive in order to sell the iMac as broken computer for spare parts.

I cannot run the recovery mode D or ALT+D (recovery mode via internet) because iMac loops and cannot load even the recovery interface.
However, it starts in single user mode: CMD+S at the startup, with command line interface.

I read the diskutil manual on Apple website, and I would like to perform a "diskutil secureErase" to the entire disk, with 7 passages DoD wiping method in order to make irrecoverable all my data.

The problem is that I cannot unmount the /dev/disk0

Here what I tried:​

/sbin/fsck -fy

mount -uw /

launchctl load /System/Library/LaunchDaemons/com.apple.notifyd.plist

launchctl load /System/Library/LaunchDaemons/com.apple.configd.plist

launchctl load /System/Library/LaunchDaemons/com.apple.diskmanagementd.plist

launchctl load /System/Library/LaunchDaemons/com.apple.securityd.plist

launchctl load /System/Library/LaunchDaemons/com.apple.diskarbitrationd.plist

sudo diskutil list

The internal disk is listed as /dev/disk0, the Macintosh HD partition is /dev/disk0s2

So I can use "diskutil" command , then

diskutil umountDisk force /dev/disk0

or

diskutil umount force /dev/disk0s1

diskutil umount force /dev/disk0s2

diskutil umount force /dev/disk0s3

I cannot unmount disk0s2 "Macintosh HD" and without unmounting I cannot use "diskutil secureErase".

Any suggestions?

Thanks.