You also need to turn off the Bluetooth, cell, and wifi modems, and keep them off.
https://www.zdnet.com/article/us-cell-carriers-selling-acces...
It wouldn’t surprise me at all if baseband processors eventually start logging and asynchronously sending gps data to the cell carriers behind the operating system’s back, so you should also keep it in a mylar sack.
Similarly the accelerometer can gather all sorts of information (passwords, text typed on the keyboard, are you driving, flying, walking, etc), so you’ll probably eventually need to keep the phone in a drawer to avoid leaking that information. Similarly for the microphone (some surveillance libraries already snoop sounds in various primitive ways).
To recap: keep it off the network in a stationary, electromagnetically shielded, sound proof box. That’s simple enough, and from there you can use it as normal.