Using Google Analytics' global site tag (gtag.js) to measure Scroll Depth - google-analytics

Question

I have currently implemented Google Analytics on a web property of mine via adding the following Global site tag (gtag.js) on all web pages:
<script async src="https://www.googletagmanager.com/gtag/js?id=UA-XXXXXXXXX-Y"></script>
<input id="user_id_val" type="hidden" name="user_id" value="{{ user_id }}">
<script>
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
var user_id = document.getElementById("user_id_val").value;
if (user_id)
{
gtag('set', {'user_id': user_id}); // Set the user ID using signed-in user_id.
gtag('config', 'UA-XXXXXXXXX-Y');
}
</script>
Note that this snippet has been enriched with user_id to enable session unification.
Next, I need to implement Scroll Depth for my web property.
How do I do that via the Global site tag (gtag.js) I am already using? I cannot find any documentation for the same. Can an industry expert give an illustrative example of how that could be done? And if it cannot be done, it is alright to simply point that out.
Postface:
Normally, Scroll Depth is measured via adding Google Tag Manager to the project. That is well-documented.
So why am I not following the recommended route of adding the Google Tag Manager snippet to measure Scroll Depth? Why am I trying to do it via Global site tag (gtag.js)? Because:
I have read that one can indeed deploy tags via Global site tag (gtag.js) as well. This official doc page explains that this is possible. So what I am attempting makes sense in theory.
I already have the Global site tag (gtag.js) injected all over my site. If I can piggy back on that to enable Scroll Depth measurement, then that is my preferred solution.
My alternative would be to replace the already configured Global site tag (gtag.js) with the Google Tag Manager snippet.
But that introduces the additional headache of how to enable session unification in GTM (another requirement of mine), which I configured with gtag.js. This transition could also break my GA. Or create duplicated effort.
Since that could overall be a messy situation needing a lot of troubleshooting, I am firstly exploring ways of just using the Global site tag (gtag.js) to solve my Scroll Depth measurement issue. Hence this question.