How to protect against thieves unlocking iPhones and stealing money from bank apps?

Solution 1:

I'm looking for techniques I can apply to my phone to mitigate any risk of my bank accounts getting wiped if my phone is stolen.

I'll restrict my answer to this, anything further would in my opinion be speculation.

Remove FaceID from all banking apps, and use a different password than the iPhone passcode. Even if they somehow figure out the passcode for the iPhone, they can't use that information to access my bank accounts.

That would be my first action. Disable any biometrics and use individual, strong passwords for each of your financial apps.

Not using any financial apps on your device at all would be more secure than using them.

Additionally, you could ensure that any account recovery methods are not using accounts or phone numbers that are attached to your device.

Unfortunately some "digital wallet" apps (similar to e.g. Paypal) protect transaction with either FaceID or the iPhone passcode

Your options are: log out after use, enable 2 factor authentication (also password protected, and not attached to the phone number of the device), or do not use these apps.