> Comments per PR [...] served as a measure to gauge the depth of collaboration exhibited during the development and review process.

That sounds like a particularly poor measure - it might even be negatively correlated. I'm worked on teams that are highly aligned on principles, style and understanding of the problem domain - they got there by deep collaboration - and have few comments on PRs. I've also seen junior devs go without support and be faced with a deluge of feedback come review time.