Yep this was the main reason for selecting Superloop as a Telstra wholesaler (and moving.our NBN onto them also, which finally got all sorted today!).
I've got the bcengi esim for the VoWifi while roaming. So hoping it's fairly seamless.
Not sure if I've got the latest message in this thread or if this has been covered, but I'm able to get VoWifi to extend to "Voice-over-2nd-eSIM-with-data" if that makes sense?
In this scenario it effectively does Voice over WiFi but over the mobile data connection of your eSIM.
From my limited experience so far, it doesn't matter how
fast or slow your eSIM data speeds are. The call quality is fine (can vary based on your eSIM's actual signal of course)
I've consistently achieved this on iOS by turning off automatic network selection when I board the plane in Oz, and asking it to connect to any network that isn't Telstra.
When I arrive in my destination country, I turn on the destination eSIM, set mobile data to route only through that eSIM, and my home sim will show "Boost using Mobile Data".

2 = eSIM
P = Home SIM (Boost)
The reason for turning network selection off in Oz is Telstra often has roaming contracts with multiple carriers overseas (e.g. Boost/Telstra connects to 2deg, OneNZ and Spark in NZ) which means the phone won't VoWiFi over the 2nd sim's mobile data.
I've managed to do the same on Android (Belong Mobile), but I haven't worked out the details. Similar approach, but the network selection trick doesn't quite work while in Oz.