Okay, so this is where I would use a Helper Table.
Have a read through that thread, and it should give you the general idea. But essentially you’d just need a separate table with 12 rows - one row for each month of the year. Then you build relations from that table and do rollups through those relations. That’s a gross over simplification, but once you get the hang of it you can use this approach to dynamically generate any sort of report for any time period that you need.