I think what we need to do is all complain in the same way, something akin to: GrapheneOS is the most secure Android option, it's just not catering to Google by sharing all user data with them. You are falling for the false argument from Google that this is about security. It's not, it's more about sovereignty and control. And you and your bank are catering to Google/the US and YOU are throwing YOUR privacy conscious users under the bus, because you do not fully understand the forces at play.
Idk, this can be much improved :)
The bank doesn't care about either. It cares about the risks, costs and opinion of the regulator on how well it evaluates and controls those risks.
If risks can be transferred to an external party and DNB signs on it, sure. If DNB says reliance on this specific external party is a risk in itself, then sure, there will be a plan for that too. The plan could be "if google becomes a problem we can publish a new version of the app using grapheneos attestation anchors in N weeks"
Or are you saying the device is insecure BUT has no local privilege escalations you can take advantage of as a user?
No they don't. At best, they do a cost-effective proxy for what should be done.