Signal data will be encrypted if your disk is also encrypted.

True.

and you don’t have any type of verified boot process

How motherboard refusing to boot from another drive would protect anything?

That’s not how this works.

If the stored data from signal is encrypted and the keys are not protected than that is the security risk that can be mitigated using common tools that every operating system provides.

You’re defending signal from a point of ignorance. This is a textbook risk just waiting for a series of latent failures to allow leaks or access to your “private” messages.

There are many ways attackers can dump files without actually having privileged access to write to or read from memory. However, that’s a moot point as neither you nor I are capable of enumerating all potential attack vectors and risks. So instead of waiting for a known failure to happen because you are personally “confident” in your level of technological omnipotence, we should instead not be so blatantly arrogant and fill the hole waiting to be used.

Also this is a common problem with framework provided solutions:

https://www.electronjs.org/docs/latest/api/safe-storage

This is such a common problem that it has been abstracted into apis for most major desktop frameworks. And every major operating system provides a key ring like service for this purpose.

Because this is a common hole in your security model.