Hey @Bijan ,
In case you aren’t already aware, this is available in REST through the transactions endpoint, though it does require an extra call to /orders/{{##order_id##}}/transactions.json. I’ll send up your feedback, but to be honest I’ll be surprised if this is implemented. The order API response can already be quite large, and this functionality is available in GraphQL as you mentioned.
Definitely understand that switching APIs is no small undertaking, however I’d consider that the effort required up-front will be far outweighed by future benefits. Especially for an analytics app, GraphQL will allow you to make connections on almost every resource, and should allow you to get the information you need with significantly fewer calls.