Apple File Vault & HD erasing
Once FileVault is done with the encryption, you can "cryptographically" and fairly instantaneously erase the drive by destroying the keys to decrypt.

I wouldn't bother setting up the next encryption. To do that, boot to recovery and do not unlock the disk.

Just repartition the drive.

That wipes the keys that the passphrase would have used to decrypt the drive. It's now cryptographically erased.

At that point, you might install a clean OS and shut down the Mac when the "welcome to Mac" screen prompts you to create an account and set up the Mac.

Shutting down then lets the next person know the Mac works, and you know it's securely erased.