Is there anything beyond NVRAM and SMC?
That's about as much as needs to be done (and indeed can be done) other than removing and destroying your old drive for security purposes.

Having said that, without specialist forensic equipment the chances of recovering data from a drive that's had a single zero-fill erase is essentially zero, and is pretty much beyond the scope of consumer-level data recovery software. The excruciatingly-long 7 and 35-pass erase routines are overkill for for most people not featuring highly on the NSA's watch list. If platters were removed from the drive and read directly by specialist equipment then limited recovery is possible, but you'd have to be a really interesting person for anyone to go to such lengths.