If that's the case then it would be down to passenger numbers and fuel costs, which way Qantas goes in the Perth v Darwin v elsewhere debate. But as things stand its clear a Perth covid-tech stop will be a less comfortable experience (unless they get some exemptions to allow mingling with Perth staff/shops).The QF9/10 is frequently operating via PER as a tech stop.
The international area of T3 would work well as a sterile transit area - at least there is a lounge. In SIN the transit passengers are herded in a separate pen to wait their flights so wouldn't be much fun there either.
On the other hand, I feel Singapore would not care about mingling, yes I've seen in the past a pen wait in Singapore. But the current designation of Australia by Singapore in the context of Singapore's current numbers and vaccination, I'd suspect Singapore would welcome an injection of Aussie currency in Changi