> The open question is supply. Finding two good used phones is easy; finding a few hundred matching ones, with healthy batteries and predictable camera behaviour, would take more work. I feel increasingly confident that it’s possible.

RaspberryPies and such are commodity hardware. You can buy one, two, ten, twenty, a hundred, and it just scales along. All of those devices will, barring perhaps a few dead-on-arrival ones covered under warranty, just work out of the box.

A used smartphone will have issues which can vary from device to device, so scaling that would take a very dedicated refurbishing setup where the quirks are ironed out (and the unsalvageable ones discarded).

I would also expect that you are starting out at the other end of the bathtub curve: with new devices you will have a few that fail soon after unboxing and using them a bit, and then they will run fine for years until you start hitting the other end of the curve and wear-related faults pop up. With old cheap phones you would start out at that side of the curve, so if you buy one hundred of those, a bunch will fail at various points soon after deploying them.

The cost of that unpredictable behaviour seems high to me. So much so that I wonder if it wouldn't be cheaper to buy a lot of unsold stock of a certain type of budget smartphone from two or three years back. Those devices could run fine as long as they aren't connected to the internet directly.