Wow, those gift voucher conditions are ridiculous. I would have thought that Qantas would be strongly encouraging people to give them cash in advance, but they have tried to be way too clever for their own good, by putting such awful conditions on the gift vouchers that it almost never makes rational sense to buy them. If I am spending my hard earned cash in advance with Qantas, then in exchange I expect to be able to spend it like cash when I come to use it (no passenger number limits, no special portal, stacking allowed, and allow them to be used for anything). Qantas should have a good look at Woolworths WISH eGift cards - they can be spent anywhere, at all the Woolies-owned supermarkets/liquor stores/petrol stations, and can be combined as desired, can be spent across multiple transactions, and they never expire - basically they're almost as good as cash, and you get a discount for paying up front. So, that's a hard pass on the Qantas gift vouchers.
So instead I took the approach suggested by sudoer:
Day 1: bought a fully cancellable & refundable flight using points on Qantas classic rewards + paying fees with the card, to trigger the 2 lounge passes.
Day 5: Complimentary Lounge passes added to my account.
Day 5: Cancelled the fully cancellable & refundable flight.
Day 25: My Qantas points were refunded. Got an email notifying me of the points refund. Still waiting on refund of the cash component. Email said it may take a further 8 weeks more.
Day 32: Fees from Qantas tickets were refunded back to the card [minus $0.40 due to card fees, I guess]. Did not get an email notifying me of the fees being refunded.
I used to buy carbon offsets a few weeks before closing a card, but because of the quite slow turn around, if you use this approach I would suggest now doing it much earlier.