that limitation is not only a UI one - the Interactive Report runtime also does not support more than 8 Group By columns. So I'm afraid this is not possible.
You might consider implementing your GROUP BY report as a classic report and show or hide this using some buttons and / or Dynamic Actions.
I think I solve it with maybe the better approach. I "normalized" the single rows with 12 columns (easier for Interactive Grid if all 12-months are in one row instead of having a row for each month) with a view that looks like
Later if there should be performance trouble I can easily convert this to a materialized view. Then in Interactive Report I did a Pivot instead of Grouping like that
The result is the same and times easier to maintain in IR-dev-configuration.