I write on my own blog and kept running into the same gap: a post would show 4,000 pageviews and I still had no idea whether anyone got past the intro. GA4 can track scroll, but by the time the number reaches me it has been through consent gating and sampling, and I wanted the raw rows somewhere I control.

So I wrote a small plugin for it. It's free on wp.org, GPL, no pro version, no account, no signup screen: https://wordpress.org/plugins/ango-scroll-analytics/

What it does: one script on single posts and pages. It records how far the page was scrolled per visit, and gives you visits, average, median and percentiles 5/10/25/50/75/90/100 on a wp-admin screen — phrased as "25% of visitors scrolled at least to X% of the page".

Three deliberate decisions, one of which will annoy somebody:

1. It measures depth through the scrollable height of the page, not the share of the article text. A short post that fits on one screen records 100%. That's why the numbers won't match your analytics tool.
2. A visit with zero scrolling still counts, as 0%. The percentiles describe everyone who arrived, not only the people who read to the end. There is a filter that drops "load-and-die" visits — page loads that never sent anything after the initial beacon, which on a live site are mostly bots and link previews — and it's on by default.
3. Your own logged-in visits aren't tracked. I added that after my own refreshing had quietly wrecked my first week of percentiles.

Everything sits in one table in your own database. The front-end script is ~2 KB gzipped, the endpoint is rate limited, no IPs are stored, Chart.js is bundled so wp-admin makes no external request, and deleting the plugin deletes its data.

Caveats, since you'd find them anyway: it went up on wp.org five days ago and I'm the only person who has used it in anger. It does no funnels, heatmaps or sessions and can't tell you why people leave. Stats are computed on the fly, so at hundreds of thousands of visits it would want caching — a normal blog won't notice.

Happy to answer anything about how the measurement works, including the parts that are my opinion rather than fact.