The main issue with old phones are NAND Flash degradation and battery problems. As long as you fix those two issues everything else is simple. These days with AI it's almost trivial to build and flash your own ROM and even the most hardcore of binary blob drivers can be reversed so long you have enough GPUs.

It's not the dark days of android where something like CyanogenMod takes a whole team of experienced system and embedded engineers to port and you don't have to hunt for information on XDA anymore.

Oh and cooling. That's something that the Pi has an edge on.

Early Android hackers might remember something called the Android Arduino Mega ADK. I suspect nowadays if you need real time control you can probably just have smartphone control the peripherals via the ESP32 using either USB C or Bluetooth. Developing over the wire protocols is trivial since LLMs can hammer out protobuf definitions like there's no tomorrow.