How do I prove my device is secure while also being able to run any software that I want?
The operating system should properly prevent software from violating the security of the system. If you mean that you want to be able to run an OS that does that provide a level of security that is expected then you shouldn't be able to prove that insecure OS is secure.
This presents enormous barriers of entry to both hardware and software entrants.
Reach out to an attestor and discuss with them what the process is for them to trust you.
>How much will it cost?
It will likely be free. If not it will be significantly less than the cost of writing an OS.
>This presents enormous barriers of entry to both hardware and software entrants
Hopefully they are high enough that fly by night malicious actors do not bother with trying to get their insecure hardware and software to be trusted, but row low enough that good actors can prove that they can be trusted.
As an industry we are getting better at security and finding and patching vulnerabilities.
>Attestation does not prove that a device meets any security bar.
But it can prove that a device's software and hardware is running software and hardware that does pass your security bar.
>No, it doesn’t provide any guarantee other than the power that be are empowered to grant themselves and their friends privileged status while leaving everybody else without a device that can run the software they want.
Security doesn't have to be perfect in order to be beneficial.