The API currently provides the following:
- Processes
- Processes Historical
- ProcessGroups
- ProcessGroupsHistorical
- ProcessReviews
- ProcessReviewsHistorical
- ProcessViews
If you look at the Processes table and then split the ProcessGroupPath into separate values you can then count how many processes are related to each group (L1, L2 or any other sub group). You can also the group structure to turn the groups into a hierarchy in PowerBI (or excel/PowerQuery) that will automatically update with the data is refreshed.
It doesnt provide the tags unfortunately. Its possible to analyse these when exporting the data from Promapp via XML file but this takes some fiddling.
Here is a link to the API Documentation for Promapp but it doesnt include a list of the tables or fields that are available from it Reporting API (nintex.com) .