brute force a bunch of inline css to change a background-image at specific breakpoints

functions.php

<?php if ( ! function_exists( 'get_attachment_sizes' ) ) :
  /**
   * Get attachments at a variety of sizes.
   *
   * @uses wp_get_attachment_image_src
   *
   * @param id - attachment id
   * @param sizes - (optional) an array of registered image sizes as strings
   *
   * @return array of urls, dimensions, and size names
   *
   * @since 0.1.0
   */
   
  function get_attachment_sizes( $id, $sizes ) {
    
    $images = array();

    foreach( $sizes as $size ) {
      
      $images[ $size ] = wp_get_attachment_image_src( $id, $size );
      
    }

    return $images;

  }

endif; //get_attachment_sizes


if ( ! function_exists( 'bg_img_styles' ) ) :
  /**
   * Get a block of responsive background image styles, scoped to the post id.
   *
   * @param $id - post id
   *
   * @param $sizes - array of registered image sizes as strings, passed directly to get_attachment_sizes
   *
   * @return string
   *
   * @since 0.1.0
   */

  function bg_img_styles( $id, $sizes ) {
  
    // get images
    $images = get_attachment_sizes( get_post_thumbnail_id( $id ), $sizes );

    // determine largest size available
    $widths = wp_list_pluck( $images, '1' );
    $biggest = array_search( max( $widths ), $widths );
  
    $style = '<style>' . "\n";

    foreach ( $images as $size=>$image ) :
    
      $media_feature = ( $size == $biggest ) ? 'min-width' : 'max-width';
    
      $style .= '@media screen and (' . $media_feature . ':' . $images[ $size ][1] . 'px) {' . "\n";

      $style .= '#post-' . $id . ' { background-image:url(' . $images[ $size ][0] . '); }' . "\n";

      $style .= '}' . "\n";

    endforeach;

    $style .= '</style>' . "\n";
    
    return $style;

  }
endif; // bg_img_styles

template.php

<?php $style = bg_img_styles( get_the_ID(), array( 'hero', 'hero-md', 'hero-sm' ) );

//I'd like to find a way to batch all these throughout the page and then output them all in wp_footer()
if ( $style ) echo $style;