Does secure erase through Terminal affect old user accounts?
Everything you describe seems to be just fine. To be totally sure I would re-enable FileVault and then cycle the keys (boot to recovery HD and erase the FV volume.) That ensures that each "block" is cryptographically scrubbed before you then re-introduce a new OS and then turn on FileVault2.

The main lesson to keep in mind is with SSD - be sure to enable FileVault before you put anything secure on the drive. That way when SSD swaps in new "excess capacity" blocks to replace others - you don't get old plain text data restored or have plain text data taken "offline" only to be possibly be put into service after you do the "wipe".

https://support.apple.com/kb/PH22241

The key phrase is:


  Note: With a solid-state drive (SSD), secure erase options are not available in Disk Utility. For more security, consider turning on FileVault encryption when you start using your SSD drive.