Defer Javascript Parsing

How do you defer parsing of javascript with Primer?
I tried several methods:
1. I’ve tried a few plugins
2. I tried adding this to header.php:

//-------defer
if (!(is_admin() )) {
function defer_parsing_of_js ( $url ) {
if ( FALSE === strpos( $url, '.js' ) ) return $url;
if ( strpos( $url, 'jquery.js' ) ) return $url;
// return "$url' defer ";
return "$url' defer onload='";
}
add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 );
}

3. I tried adding this to functions.php and replacing with the correct URL.

	<script type="text/javascript">
		function parseJSAtOnload() {
		var element = document.createElement("script");
		element.src = "somejavascriptfiles.js";
		document.body.appendChild(element);
		}
		if (window.addEventListener)
		window.addEventListener("load", parseJSAtOnload, false);
		else if (window.attachEvent)
		window.attachEvent("onload", parseJSAtOnload);
		else window.onload = parseJSAtOnload;
	</script>

I’m using Stout, a Primer child theme. But even with just Primer I get this issue. Though I did not try the plugin method with just Primer. The issue goes away when I change to the Twenty Twenty theme.

Has anyone successfully defered parsing of javascript with Primer?

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