> the issue is that when a phone is unlocked, they can just plug in a USB device and scrape everything off it

That's useless if the backup account and the main account use different keys for the home partition.

VeraCrypt (used to?) have this. It was called a hidden volume. One volume, but two keys, two passwords, and two different containers full of data. Technically, the second volume is written into the partition "from the back" using key 2, while the first volume is written "from the front" using key 1.

Fun fact, there's no protection against writing over data in the other volume if volume 1 + volume 2 exceed the size of the partition - and there can't be, otherwise the volume wouldn't be hidden.