phones are the most sensitive devices we own, and should have the tightest security. We’ve see android fail in its compartmentalization many times, to say nothing of its built in limitations.
as a result, malware sneaks into the android play store so often, its become an exercise of bored tech writers to troll them by doing it intentionally so they write some click bait about it.
Since your playing apple with this this phone (designing the both the hardware and software) and have experience with qubes, why not go in that direction? or, at least something like that under the hood. the design of xen on arm is cleaner than it is on x86. but, failing that, you could at least have containers emulating a full os so users can partition things and try apps, or even open a pdf or movie, without less chance of compromise.