Don't forget wattage requirements. Compute per watt is by far the most important factor. I would be curious if resoldering many phone CPU chips to make some sort of computer is power competitive with relatively modern cPUs.
We are talking about reusing for old phones here- does it really need to be power competitive? When we talk about compute per watt we are really talking about cost. What is the cost of something modern and power competitive compared to reusing a phone that was already purchased, in a setup that mostly idles?