iMac has High Sierra, need help erasing my iMac

Hi. I just noticed that these instructions (What to do before you sell, give away, trade in, or recycle your Mac - Apple Support) do not include a secure erase (i.e. overwriting every block on the drive.) After an Intel Mac's drive was erased this way, I'd expect that data recovery software can often read the blocks directly and find file content and sometimes even metadata, no? (The formatting doesn't take long enough to write to all blocks.) I'm wanting to prep two Intel macs that can't run the latest OS for sale, each with multiple partitions, and one has a third party hybrid drive; the other an SSD. Erase and reformat a storage device in Disk Utility on Mac - Apple Support clearly says "Secure erase options are available only for some types of storage devices." I'm not seeing the secure erase options. Am I missing something or does following Apple's instructions result in the computer that's handed over still likely to have recoverable user content on it? I'm thinking I need to resort to something like dd if=/dev/random of=/dev/foobar bs=1m in Terminal.

Wow, so I'm right - mostly - there's a risk, but it's smaller than I thought. Per https://www.cleverfiles.com/help/ssd-drive-data-recovery.html, l I need to check if the SSD is old enough that it didn't support TRIM. I remember that in the early days of SSDs, TRIM support was rare.

(Hmm. Post button isn't working. Blacklisted URL? No, login timeout is very short.)

Follow the instructions in this Apple document: What to do before you sell, give away, or trade in your Mac - Apple Support  

https://eshop.macsales.com/blog/44781-how-to-securely-wipe-the-data-stored-on-a-drive-in-macos-high-sierra/

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SSDs does not need secure erase by their nature. You'll notice the for SSDs Disk Utility does not offer that type of erase.