Huawei Mate 20 Pro, Optus branded/stock but unlocked. Considering $599 is a mid-range type of price, the specs (battery in particular) are really good, I think. Plenty of stock in SA it seems, hopefully same elsewhere.
Edit - just letting you all know the only Optus app pre-installed on the phone is MyOptus, which can be uninstalled. The phone also updates to EMUI 10.