The thing is you would only turn this device on when you need to sign the tx. The tx you can create on sentinel. So highly unlikely to be tracked. You can keep it in a Faraday bag, you can live in a Faraday cage!!! There will always be doubts. What if the chip you use on your foundation device has been compromised? Maybe your Jade isn't as honest as you thought? Even building your own doesn't guarantee complete security. I have often said we over complicate storing and sending Bitcoin. Common sense and good practice is a start, to build on. If you believe all Pixel phones are waiting for Goolag to press a button and steal your data, then that's what you believe. If you think the ROM you flashed isn't what it seems, write your own and overcome those problems. Or, use a paper wallet and look after it