I've done my own research & got some f'back from a Melbourne car audio installer that yes, there is some potential to upgrade the audio system in this model. Bad news/good news. The bad news is the factory infotainment unit can't be ugpraded. The good news is that firstly, Toyota advise me they're adding Apple CarPlay around November via a firmware update for existing vehicles, and standard on new vehicles sold thereafter.
Secondly, my car audio installer advised I can use my current Focal Utopia 165W-RC passive front splits & Alpine MRVF-545 amp. He also said I can use some good 6.5" coaxial speakers in the rear, sound deadening in the doors, and in lieu of an aftermarket head unit - use an Audison Bit Ten digital processor (see pic below). It has a very impressive suite of features & is a God send really.
I'm still mulling over rear speaker options, cable kit & choice of sound deadening material though. So if you have any suggestions, fire away!