How to "securely" erase my MacBook's SSD
You don't need to.
Once you erase an SSD on  a Mac, it's unrecoverable even from a forensics standpoint.  This is because of the type of TRIM command that the hardware (SATA controller on the logic board and the SSD) issues.  Basically, if you were to stop an erase mid-process you theoretically could take it to a lab or the manufacturer to recover data.  So, it's not impossible but highly, highly improbable recovery is possible. I've previously written about this topic:

Can the data on an MacBook Pro SSD be recovered after formatting using Disk Utility
How to get the &quot;securely erase&quot; function of Disk Utility on El Capitan &amp; Sierra
https://apple.stackexchange.com/a/288756/119271
Recovering files from SSD and SSD life?

If you want to be even more certain, turn on File Vault (Apple recommends this) when you reset it.  Then disable it (for the next user).