That's the weird thing about DAX / PowerBI... the reports are fully interactive. You can get data from a stored procedure or view that has no aggregates at all, and do it all in DAX. It uses something similar to column store to compress the data so you don't really have to preaggregate it. Most of the time you wouldn't drill all the way down, but you can. And then export that to somewhere.
Silly me. I think a big part of the OP's problem is that he has an OLTP structure, and he needs a Data Warehouse/Star Schema structure. I should have mentioned that a long time ago. Then he has a Forecast fact table, a Sales fact table, and then dimension tables, like Product, Store, etc.
Little wonder he's confused. I would be too.
How soon does all this have to be done? Creating a data warehouse / star schema for this may take a minute. Some CREATE TABLE scripts for the data you already have would help a lot. With that we can figure out what your star schema could/should look like, and queries to populate all the tables. And then you can start writing DAX for the analysis...