For your use case, why not simply put the categories in the `Categories not to indent’ box?
I agree, it would be nicer to just indent subcategories. However, at the time @Lilly developed this TC that was quite tricky to achieve. Personally, I find the above solution very workable unless your categories are unusually dynamic.
While I’m here…
Feature Request - expandable / collapsable subcategories
This is discussed here:
I do wonder if it would work as an extension of this Theme Component.
We could potentially have some budget to go towards this if anyone is interested in joining forces.