Update on this if anyone's interested: with a week left to book with my travel voucher (not gift voucher), after checking twice with their live chat that this is possible, I booked a return Max Bundle within a dollar of my original voucher cost, and then tried to immediately cancel it. This failed, probably because it hadn't ticketed yet, or something else more arbitrary within Jetstar's booking engine.
I had to get on live chat again to fully cancel it but now have a new voucher on its way for the full amount, this time with 36 months expiry, and no longer have the threat of the Per-Syd redeye with a toddler (which was my alternative option to using it up). So, a success of sorts, albeit one with the continuing threat of a longish J* flight in the future.
Edit: it arrived a day later, expires Feb 2022!