Is it possible that QF could take some A330’s from domestic trans-con and use them elsewhere give they no longer have VA A330’s to compete with?
Yes, as RyanairGuru noted this is already happening. There's no reason for Qantas to run A330s east-west unless there's high enough demand to fill those larger aircraft, because as you say they no longer have to compete against Virgin's A330s, which is a good example of how competition really does raise the bar.
As Tullamarine said, the A321 freighters will likely mean that the level of A330 capacity to Perth never recovers to pre-pandemic levels. With Virgin only operating 737s there’s no competitive need to offer a premium product, and cargo can increasingly be moved by dedicated freighters.
My personal prediction for the Perth market is that going forward there will only be one or two A330s per day to maintain a competitive edge, capacity discipline through the use of 737s will push up average Qantas fares, and then Jetstar increase frequency to 2 or 3 daily to pick up leisure passengers priced out of flying Qantas and to keep Virgin on their toes.
In some ways, I see this approach being replicated across the country. Until the next 3 787s are delivered, capacity growth is coming from adding QQ E90s and returning A380s to service. The E90s backfill for larger 717s, which replace 737s in some markets, freeing up 737s to take over domestic (and Tasman) flying previously operated by A330s, in turn allowing for some incremental international growth. Over all, Qantas reduce domestic capacity, leading to higher fares for people who will pay a premium to fly Qantas. Jetstar can be used to target the lower yield passengers pushed out.