Painful experience dealing with and flying Qantas

Where to start with this very long, complicated situation. We used flight credits to book flights from Wellington NZ to Johannesburg South Africa as previous flights booked in 2020 were cancelled due to the pandemic. Firstly, there was no way to book a flight on the website, we had to call to redeem the credits and book our flights. After 3 separate calls, and failed attempts to book these flights because we kept getting told the flights were booked, only to call back days later to find out no one had booked the flights, we were then told we would need to pay a shortfall of thousands of dollars. When I queried this they had accidentally forgotten to apply the credits, and then wanted to charge me $70 per passenger because I had made my booking over the phone. I again queried this fee as we were not given the option to book our own flights using our credits. This fee was eventually waived. Once the booking was eventually confirmed, we then found that we couldn't manage our booking online and had to make several more tedious calls to allow us to finally have access to our own booking to select seats, but as the date of our trip got closer we kept trying to select seats and the only option given was to select a seat preference, window or aisle. So we selected our preference, tried to check in online the day before, online check in was not available. We figured we could select seats when we checked in at the airport. Unsurprisingly, the flight was full and there were no seats to select, so we'd have to deal with whatever we were given. When we boarded the flight found that we were seated in the most awful seats, our preferred seat selection was ignored and we had the joy of being seated in a row without a window, next to the toilet. The seats on the newer planes are ridiculously small, I was crammed into the non-window window seat with no space to move, claustrophobic and in pain the entire 14 hours. I have massive black bruises all over my thighs from being squashed against the armrests of my ridiculously small seat. My husband's Samsonite suitcase which has been used once was also broken when it arrived in Johannesburg. So we decided to upgrade our seats to premium economy for the return flight which is 3 weeks away. Despite being reluctant to give Qantas any more of my money, for such a long trip and comfort we opted to suck it up and do it. We can't. We have called Qantas twice in 24 hours to try to upgrade our seats, but as they are connecting flights, and the shorter of the flights between Sydney and Wellington doesn't have a premium economy offering, the agents can't upgrade a segment of a connecting flight. We opted to escelate the call to the reservations team, on the advice of the agent assisting us to find out if they might be able to help us upgrade our seats. I was told they would contact me. We also put in an escelation for my husband's damaged luggage. I received an email a few hours later advising me to contact the reservation team on a number I cannot call from South Africa, as they as customer service agents cannot process this request. I rep**** asking for clarification as I was told I'd be contacted by someone to assist. I received the very same copied and pasted response to call 131313. I called Qantas again to try to find out what I should be doing next as the email response was vague and didn't answer my question. The agent assisting then told us she could process an upgrade, and told us there would be a $5000 (yes five thousand) shortfall PER PASSENGER to upgrade to premium economy, in addition to what we'd already paid for our original flights. We then asked if we could cancel the flights, get credits and re-book ourselves in premium economy, as if we did it that way, we would only have to pay $7000 for both of us combined. We didn't actually get a definitive answer to this question. No resolution to our query, and dreading the prospect of another long haul flight with Qantas.