Really interesting question. I don’t know of any HTTP Archive indicators for DOM activity between the initial parse and another timing metric. And I suspect that many non-client-side pages will still have a noscript fallback.
Another thought I had was to look at DOM elements at DOM Content Loaded vs onLoad vs Fully Loaded and try to look for patterns. But HAR files don’t collect that level of detail.
Could using Wappalyzer to analyze for the presence of certain frameworks be useful here? Or useful combined with another heuristic? Using Wappalyzer to Analyze CPU Times Across JS Frameworks
Just thinking out loud in case it helps…