My plan is to have it just run off of an esp32 and run a 2.5mm cable to my dslr to control the shutter. I’m sure that someone sells a shutter button for the iPad that you can rig up to something to trigger it. There might even be a way to use Bluetooth for it.

You don’t even need to use a pi. Pretty much any SBC should be able to run it, but an esp32 should be sufficient for a simple scanner, especially if you’re not uploading the pictures from the scanner