Remove required on bank of account and autofill it
The bank account requires a bank only for the SEPA payment. So it can be a constraint when user has only the account number but no information about the bank it is linked to. So I propose to remove the required. But for some countries the BIC can be deduced from the IBAN number thanks to library like schwifty. So I propose to also fill or create the bank field for the account for which we can deduce this information. And also to check that the BIC filled by the user is also correct. Also by definition IBAN is a unique identifier so a bank account could have only one and the number must be unique.
I think this provides a flexible and good user behavior for the user.