Hey @Jess_171
I don’t tend to worry about complex topic structures as I find that most visitors don’t use them to discover content. I suppose this is because topics don’t answer a specific question, whereas a Google search will.
Case studies are a different matter and do benefit from being filterable, by vertical for instance, but a single layer of topics is normally sufficient for this.
Monitoring what your visitors do is a great idea. I suggest using Hotjar heatmaps/recordings as well as Google Analytics and Page Performance in HubSpot.
Good luck 