Filter menu items and replace their default output with custom WYSIWYG content

add-content-to-menu-item.php

<?php
function add_content_to_sub_menu($item_output, $item, $depth, $args) {
	// Checks theme location (remove to apply to any menu)
	if( $args->theme_location == 'primary' ) {
		// Get Content from ACF Field
		$content = get_field('content', $item);
		
		// If there is content, replace the link's regular output
		if( $content ) {
			$item_output = $content;
		}
	}
	
	return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'add_content_to_sub_menu', 20, 4);