Use GraphQL or Really stick with RESTful routes. The more predictable the schema of these Dictionaries/hashes/JSON are the less likely you are to see that mess above. This is true whether you are using a FP approach or an OO approach. Using an Imperative coding style when doing ETL will always be hairy.

That function also violates the Single responsibility principle. I wouldn't even know where to begin to write a unit test for that other than breaking it down into smaller parts. There are design patterns that could be followed in dynamically typed languages that would avoid that mess altogether other than just OO.