I don't mind the addition on the caret, especially if it is customizable with a CSS class. I would recommend:
- using the darker color instead of white
- only changing the indent of this label, not pushing all the sub folders over (I see you had it like this in your proposal here, but wanted to call it out) I think this would address @tessus real estate issue?
One other idea I'd like to add – and it might solve this issue of "mental real estate" in a different way – is to have a count of items next to the type. So "Notebooks (25)" or "Tags (130)". This would remind you that something is there, even when it is collapsed.