Just to add some closure to my issue. I spent a week calling the call centre trying different variations of my proposed additions. Unfortunately the overseas call centres could not reprice the itinerary.
I reached out to Qantas via FB Messenger explaining my predicament and asked if they could identify the possible cause. After some back and forth comms, they passed the request onto the right area and someone contacted me via email advising there was availability and quoted the taxes and charges. Payment was taken over the phone and the ticket was reissued with 5min of the call ending.
BKK-MCT-TAS and MCT-BKK flights were added to my itinerary, the points stayed at 318k and the 5k change fee was immediately deducted.
Note that comms with Qantas via Messenger isnt instant - it can take 1-2 days to receive a response. Overall it took nearly 2 weeks for my issue to be resolved via Messenger. It worked in this instance because there is alot of WY availability 10 months out and there isnt a huge demand.
This will probably be my last OWA. I was lucky I had a complete itinerary and just wanted to tack on a few extra flights to maximise the award. Compared to my last OWA in 2024, the lack of awards released on a reliable schedule makes it difficult to plan. Back then, JL seats were released 360 days out. Now its really only AY and MH.