Welcome to AFF
Not sure if you are using a flight credit (with the same booking number as the original booking) or a TravelPass (some flight credits have been converted to a TravelPass (which have been generally easier to use).
Based on what you have stated, I suspect its flight credits.
One condition of the flight credits is that you need to choose the same or higher fare bucket (you also need the same passengers names, but I believe you can have a different destination). This might explain why the flight credits cannot be used for Economy (despite what has been initially stated)
Unfortunately, now I think fuel charges will be going up.
I'd call Qantas again (yes its a long wait and most of the call centre staff are not that helpful) but you need to find out what solutions they are able to actually provide (ie what can actually be booked at that moment or maybe whether you can convert your flight credits into a TravelPass).
If you aren't able to get the same type of itinerary, perhaps maybe consider trying to use the voucher for one of the two departure points (ie London or Germany)) and then book the some other transport for the other destination (eg ground transport or maybe air transport with wide margins for changing airlines or having a second separate booking). Not sure for example whether flight credits can be used for an Qantas/Emirates codeshare flight from Germany via Dubai to Sydney.
You will have to persist with Qantas Call Centre.