Choice of device is mostly useless. They're all the same re. Security. Which is to say their only as good as the jurisdiction their being sold/used in. And given the laws Australia has implemented, if a company sells their product here or has operations registered in Australia you can't guarantee it hasn't been backdoored.
Security is only as good as the weakest link. Each company that touches the phone or the software increases the attack surface.
Therefore, from a security perspective, the phones with the best security are the iPhones and the Google Pixel phones. They also generally have the quickest security updates.
If you install a custom ROM (e.g. LineageOS), you can make almost any phone reasonably secure, but that involves a lot of time and hassle. If you don't trust Apple/Google, that's probably your best option.
An alternative approach is to have a second device for security sensitive purposes, which can be locked-down, only essential apps installed, offline by default, etc.
> f you install a custom ROM (e.g. LineageOS), you can make almost any phone reasonably secure
Do the official LineageOS releases have Verified Boot (allow you to lock the bootloader)? Everything I've tried uses userdebug keys, which is pretty bad security-wise, and requires an unlocked bootloader.
I ask because Verified Boot ensures the root of trust is there in the binaries. If they're wrong by one bit, the OS won't boot.
Realistically, even if your OS has additionally security features compared to AOSP, it'd still be like building a house out of obsidian, then putting a wooden barn door on it.
At that security level, you can't trust a single device, and you'll probably need to look at security solutions such as hardware security keys (Yubikeys), Shamir's Secret Sharing, plausibly deniable encryption, etc.