1118517-mi-gtag-set-custom-dimension.php

<?php // Ignore this line when copying/pasting this code to your child theme's functions.php file.

/**
 * This example sets custom dimension #9 to be the WordPress
 * post ID of the page/post being visited.
 *
 * This is compatible with MonsterInsights gtag code. This filter supercedes 
 * the monsterinsights_frontend_tracking_options_analytics_before_pageview
 * filter.
 *
 * - Change the number 9 to your custom dimension index number.
 * - Change the call to get_the_id() to what you need the CD value to be.
 */
add_filter(
  monsterinsights_frontend_tracking_options_persistent_gtag_before_pageview,
  function( $options ) {
	  $options[ 'dimension9' ] = get_the_id();
	  return $options;
  }, 
  20 // priority
);

Use the following to execute a JavaScript function instead of a static value:

add_filter(
	monsterinsights_frontend_tracking_options_persistent_gtag_before_pageview,
	function( $options ) {
		$options[ 'dimension9' ] = '\' + getCustomDimensionValue_JS() + \'';
		return $options;
	}, 
	20 // priority
);

Thanks for sharing your solution, @ThePiGuy :-)