Welcome to the forum! (BTW - great username. I loved those books as a kid).
Ultimately, everything you’re looking to do are things Power BI does really well with pretty minimal effort, as long as your data model is properly structured.
The things that immediately jump out to me here are that:
- you definitely don’t want to split your sales table (i.e., fact table) by quarter. That is going to make your data model unnecessarily complicated and your DAX measures much more difficult.
- to get time intelligence analyses (such as Q over Q) to function properly, you will need to add a proper date table and mark it as such
- you’ll want to combine your multiple accounts tables into a single proper lookup table (i.e., dimension table )
- finally you’ll need to establish the proper relationships between your fact table, date table and other lookup tables
The place I would start is to go through the following course in detail, which will guide you through the necessary data transformation and data modeling concepts and procedures necessary to accomplish the steps above:
Then, once you’ve got your model set up correctly, these videos should help with the specific analyses you want to do:
See how this goes for you, and if you run into any problems or questions just give a shout out on the forum - tons of knowledgeable folks here eager to help.