@Anteaus,
I don’t know all of your queries requirements but you may consider duplicating the data (keep it both in Report’s collection and in Financial’s collection).
In general, the data structure should be based on your queries.
For more information I recommend you to look at this course: MongoDB Courses and Trainings | MongoDB University
or follow this guide: https://university.mongodb.com/exam/guide#data-modeling-developer-only