I think you are thinking about air gap devices all wrong.
The threat model of an air gapped device is it is too dangerous to be connected to ANY network, because advanced threat actors WILL try to sneak in. For that kind of model, you need to think about the set up from all angles.
You said you will use USB data transfers to send data. Who will verify that the USB device is not tampered and armed? The threat model requires ANY sort of connectivity to be verifiably vetted to be safe. Also regarding the need of a smartphone, if you install multiple apps, can you verify all apps you install to be not Trojans? What about the physical security? Can the threat actor hit you with a tire iron and collect your device anyway?
A correct set up of an air gapped device is:
- The device (say, device X) has a stripped down operating system based on Linux or BSD or similar which has minimal amount of functions which is specifically set up to do what you want to do and nothing more
- The primary disk of X is set to be read only. You set up another device Y that is a copy of X but it's sole functionality is to securely verify that the data in X 's primary drive is in a known safe state and is not tampered or corrupted. The verification tool must be hardened as possible.
- Both devices is always in an power off state, with both drives are separately stored in a secure physical vault. The device X and Y is also stored in it's own secure room with high physical protection.
- Any electronic device that goes inside the secure room must be checked with state of the art malware detection mechanism. This includes everything from a USB device to a digital watch.
- Only authorised people must go in and come out with full body checking
Pretty sure unless you are Mr. Trump or someone on behalf of him, you need a truly air gapped device. All other "air gapped" devices has flaws and if someone is willing to use a zero day on you, someone is willing to defeat your air gap. Sometimes a simple tire iron would be all that is needed.
Edit: for your actual use case, get a normal GOS device, keep it up to date. Disable network connectivity on your apps and don't put a target on your back.