How to decompress DataModel from PBIX file.

So far, the best approach I've found is to manually use the tool "ALM Toolkit". Open your desktop file in PBI Desktop, and then open ALM Toolkit. You can choose the desktop file from here as "source", and the online version as "target". Compare differences just to run the app. Then, even if everything is identical, you can export to XLSX the full definition of the PBIX. Here you'll find JSON formatted cells with things like your formulas and column names and the works. This will become a weekly effort for me to re-run any recently modified PBIX dataset sources, and generate new XLSX files that are then my source files for another process to consolidate and report out my data dictionary. Being all of this is running via APIs to PowerBI Desktop/Online I have to also imagine I could write something directly to the online service to pull the same JSON data dictionary. However, I haven't found the correct API yet.