So true. And next to the 675,000 lines of business logic that you describe, you'll need an additional 133,700 lines of boilerplate code that:

- ensures all operations are atomic. You can't just post to an API and then insert something in your database: that's a guarantee to have payments in one place and not the other.

- has some way to retry on failure: your average job-queue with exponential backoff is quite certainly still too naive.

- has full, secured and guaranteed audit logs. That have all the data needed for an audit, but also not too much. You chose to not go for the Event Sourced Architecture because of Reasons? Good luck bolting it in now.

The hard part isn't generating some pain.001.001.03 (yes, that really is the name for the SWIFT Payments Initiation in iso-20022) format. The hard part is everything else.