Your security model is non-standard, and most people won't go as far as you to transact with and store their bitcoin. But if we go back to the hardware of your device, you again don't know anything about it. What do you know about the circuits, electronic components, and the various connections inside your hardware? Do you know all their functions and how they interact?

As an example, the security team of Kaspersky randomly came across a hardware backdoor and hidden vulnerability inserted in five generations of Apple CPU's that are used in many of their products. They were investigating another vulnerability when they accidentally came across that one. It allows for complete control and access to Apple products remotely and uploading malware, spyware, and whatnot. I am just mentioning it as an example of what developers and engineers can do or what they have to do if someone requests it from them.

Very few systems (if any at all) can be considered impossible to penetrate. It's just a question of how much interest there is to break them and who is doing the breaking.