Ok, bear with me here because it's just a concept and I'll never actually build it. The key is that it is two computers in one. One operating fully connected to the internet, the other is not. The first being a streamlined phone capable of custodial #bitcoin purchases and sells running a bitcoin only software and application. The other being an air-gapped signing device that only signs transactions. It does so through two lasers that can transmit data but not be intercepted remotely in any way as they are close proximity and enclosed in the case. The lasers send the transactions and the person signs for it by manually entering their words or using the camera and a qr code. The seed words are not stored on the device similarly to seedsigner. Oh and the device has biometric scanning of course to unlock.