Boot the Mac as you normally would and turn on FileVault. Once it is complete, boot back into Recovery Mode and wipe the drive. Goal achieved.
I was going to post the following and then I found your post. Is it really this easy???
I have read that rebooting in Recovery Mode and erasing one's internal SSD does not truly erase it and with the right equipment, data could indeed be harvested from an SSD allegedly erased in this manner.
In the case of a HDD, when you choose Erase, you can decide how thoroughly you want to erase that HDD. This is not offered for SSDs, I believe, because it shortens their life span and because it is does not really work.
I am a volunteer who is collecting old Mac computers from folks who have newer Mac computers but have never disposed of their old one. I am collecting them to give to a charity. I want to be able to honestly tell anyone who donates a Mac computer that I will see to it that all their data is securely erased from the MacintoshHD on any Mac they donate.
I am almost positive there is some way I can use FileVault to encrypt the MacintoshHDs of any computer they donate then treat the encrypted drive as if I had lost the password and the recovery key thus preventing any data on their drives from ever being harvested.
Does anyone know how to do that?