Calling Boursobank and asking them to “retirer mon appareil de confiance” lets you validate transfers and 3D Secure payments using an SMS and email code instead !
It took less than 10 minutes in my case.
Here is a direct link to request a callback from them, which also avoids having to go through the chatbot.
Just be aware that logging back into the app (after the Face ID and identity verification) automatically re-enables the trusted device and disables the SMS/email authentication again.