I should be able to tell my bank to only trust devices running an OS signed by the grapheneos key, and more importantly I should be able to tell them to trust an OS signed by my key.

How do you know that your OS installation doesn't include malware? Like there have been many cases in the last few years where npm modules were found to contain malware. Who says that's not also the case in some modules that are a part of your OS?

And more importantly, who is legally liable if malware actually does cause harm? E.g. malware acts on your behalf and sends your money to some criminal organization. Not only did you lose money, but now you're a suspect of supporting a criminal organization!

Of course that issue might be alleviated if you simply don't have any money to send anywhere in the first place. That might be a viable alternative, but it only works for some people, i'd say. Or you could also set a daily transaction limit of say $100 that you can use to buy groceries; to limit your losses that way. The limit ofc cannot be changed from your phone alone, you need to go to a bank physically to change it or sth. Otherwise malware could again change it on your behalf.