To me, this is hard to answer without knowing how you intend to use it.

My useage, which helps me immensely, may be irrelevant to you.

>>>What metrics would you suggest to justify the ROI?

First of all, have you defined the problem you're trying to solve? What
issues do you currently face?

Or, do you have a solution in OneNote but you haven't yet identified a
problem that needs solving?

If the answer's not obvious to you then perhaps none exist?

Metrics such as time and cost immediately spring to mind. But also
procedural improvements to make workflows more efficient.

--
Regards

John Waller