I'm not on a 5G plan so I can't say for that, but 4G works flawlessly. VoIP is supported. I am also on Amaysim.

I paid €42 EUR for shipping from Europe.

No hardware or software problems related to Australia.

I'm running /e/OS. It was more expensive than paying for Google Android then flashing /e/OS myself, but I wanted to support a non Google option.
Banking apps and authentication apps don't work on default /e/OS because they rely on Google Play Services. You can add play services back, but that defeats my reason for using /e/OS.

I love the physical switch. I remapped it to the torch (after 1 OS update) and use it all the time.

I am slightly disappointed by the case. I was hoping it would cover all of the back and sides. It does protect from dropping on a flat surface, but I'm worried about a table corner or gravel scratching the unprotected section.

I'm not a big fan of the apple style home screen. I would prefer to hide and/or uninstall apps that I won't use. The best I've got is putting them in a folder. Overall it's still usable and not a deal breaker.
Edit: just discovered lawnchair. I'm very happy with it.

Overall opinion: excellent phone, would buy again.