Actually DL Diamond can be earned without ever flying on DL I am PM/MM and every other year DL gives me Diamond to tempt me to fly them again -- to no avail. DL has had "secret" level higher than the top FF grade for the past 25 years (going back to the infamous FC or Flying Colonel status ... ask any FA on their transpacific flights if you don't know ).

Bottom line is you won't get a match with QF, and DJ doesn't provide any reciprocal benefits on Oz (DJ) flights. Which radically limits the usefulness of any DL status unless you spend a lot of time in USA. QFF Gold is worth getting as it gives lounge access of considerably better quality than the DL lounges.