@madrooster thanks for your insight. Seems QR has tightened their availability for QF customers since last year as I had much less trouble then - could almost take my pick of departure days when looking at a similar time of year.
Anyway, do you think there's any prosepct in your experience of QR improving availability closer to the travel date (May 21)?
QR does release additional award inventory closer to travel if loads are low. I would not recommend holding out for it though, if it's an important trip.