I will add to the welcomes to the forum,
@david h
As others have already pointed out I think you're the victim of confusion and possibly misleading information - but that may also be the result of the agent(s) understanding of the nature of your request.
First and foremost, when you say you've booked economy tickets on the QF53 (BNE-SIN), were these Sale fares? Flexible Economy? The type of fare is very important (and I know this can be extra confusing). The cheapest sale fares, for example, are not upgradeable with points at all, so hopefully you have not been caught out by this (when booking, the QF website does show which ones are upgradeable, but you have to look in the details for this information).
Assuming you have an upgradeable fare, then the cost to upgrade to Business Class will be either 54,500 (per passenger) from discounted fares, or 29,900 pp from Flexible Economy fares. This flight is usually flowin with an Airbus A330, so there is no Premium Economy available.
However, for two passengers, the chances are likely slim to get both upgrades without Qantas status.
I feel this is what some of the agents were trying to express to you - that the better chances for upgrade go to the higher status levels such as Gokd, but more likely Platinum level flyers. Reading between the lines, it sounds like perhaps the agents were telling you that the particular flight you're booked on is filling up with people who have booked Business Class already, which reduces the number of seats available for those seeking upgrades.
I understand you may feel this is a "joke" but this is how just about all the loyalty programs work.
Further, the only way to ensure you are upgraded right NOW to Business Class, is to pay the cash difference on top of the fares you have (which I suspect is going to be quite expensive), otherwise you will koin the "lottery" as we call it as upgrade requests are processed in the final days before the flight.
I hope the comments in this thread, and resources provided, have helped you to understand more about the process.