From an opsec standpoint, would leaving the phone in airplane mode and turning off location services effectively block access to the phone?
From my understanding airplane mode disables the sim, wifi, gps, and bluetooth entirely, but it's possible to re-enable wifi, gps, and bluetooth. It's something I got into the habit of doing because my phone searching for 4g cellular data ate into my battery.
For the camera your best bet is to just put a physical cover. There are a lot of sliding adhesive ones you can buy online that are convenient. I stick them on laptops as well, especially work-issued laptops.
For the microphone ... ideally build and install LineageOS yourself on an Android device. Don't trust iOS or Google's official Android builds, they have closed source software that may have France-specific backdoors.
From what I've read, the current theory is that access is initially possible using the cellular modem. Building and running a custom Android wouldn't necessarily help in that case, because the modem/baseband firmware is completely isolated and runs independently of the OS.
It seems like general camera/audio recording would need a secondary exploit.
Hm. I wonder how much work it would take to desolder the microphone and add a physical switch for it. Or maybe a magnetic reed switch in series with the microphone so I don't need to drill extra holes in the phone enclosure -- just hold a magnet up to the phone when I want to use the microphone.
From my understanding airplane mode disables the sim, wifi, gps, and bluetooth entirely, but it's possible to re-enable wifi, gps, and bluetooth. It's something I got into the habit of doing because my phone searching for 4g cellular data ate into my battery.