How to securely erase disk & reinstall Catalina?

Once you boot into Recovery Mode you are running from a virtual installer drive so it is safe to erase the entire drive. Even if you could not, then you could just boot into Internet Recovery Mode. If the local recovery partition is missing or damaged, then Command + R will boot into Internet Recovery Mode instead.

If you want to zero out the hard drive, then you want to use the secure erase option if it is still available in Disk Utility. Otherwise you can try using the command line utility "diskutil" to see if it will allow you to zero out the drive with a secure erase. If not, then you can use the "dd" command to write zeroes to the physical drive.

Another option would be to first enable Filevault and let it finish encrypting the drive. Then erasing the drive normally will destroy the encryption key so any information on the drive will be inaccessible.

If you want to be safe, then create a bootable macOS USB installer.

Edit: I recommend selecting the physical drive when erasing the drive. The physical drive will appear something like "Apple HDD....", or "ST1000xxxx ....", etc.

Very simple process.

Reboot the iMac and hold down

Command R

Once you are in the Recovery Partition, open Disk Utility.

From the view menu at the top select "Show All devices".

On the disk utility menu select the top level drive above Macintosh HD (APPLE SSD XXX). Click erase.

Or you can just select Macintosh HD and click erase, make SURE you click "Erase Volume Groups"

When complete all data will be gone, you can quit out of Disk utility and then can select Install macOS.