For REST API, when requesting a post, create an array to the blocks and also evaluate the ACF fields

my-plugin.php

<?php
// This doesn't really work just yet
add_action(
  'rest_api_init',
  function () {

    if (!function_exists('use_block_editor_for_post_type')) {
      require ABSPATH . 'wp-admin/includes/post.php';
    }

    // Surface all Gutenberg blocks in the WordPress REST API
    $post_types = get_post_types_by_support(['editor']);
    foreach ($post_types as $post_type) {
      if (use_block_editor_for_post_type($post_type)) {
        // https://developer.wordpress.org/reference/functions/register_rest_field/

        register_rest_field($post_type, 'contentraw', ['get_callback' => function (array $post) {
          return $post['content']['raw'];
        }]);

        register_rest_field(
          $post_type,
          'all_blocks', // don't call this 'blocks' or else you can't edit them. I don't get it either.
          [
            'get_callback' => function (array $post) {
              // get the blocks
              $blocks = parse_blocks($post['content']['raw']);


              // Remove the null blocks - why does wordpress have these??!?
              // This breaks the WP editor - do not do this...
              // Will have to filter this out on the frontend.
              // $blocks = array_filter($blocks, function ($var) {
              // 	return !empty($var['blockName']);
              // });

              // run get_field on acf fields
              foreach ($blocks as &$block) {
                if (array_key_exists('data', $block['attrs'])) {
                  foreach ($block['attrs']['data'] as $fieldname => &$value) {

                    // $block['attrs']['data'][$fieldname] = wp_kses_post(get_field($value));

                    // if _fieldname then do fieldname = get_field(value)
                    if (is_string($value) and str_starts_with($value, 'field_')) {
                        $realfield = substr($fieldname, 1); // remove the underscore to get the real fieldname
                        // get_field() always produces null
                    	$block['attrs']['data'][$realfield] = wp_kses_post( get_field( $block['attrs']['data'][$realfield] ) );
                    	// $block['attrs']['data'][$realfield] = "this will work, above will = null";
                    }

                    // Alternative take: using Wordpress functions work
                    // So now we have to enforce rules on field names so we know what to do:
                    // ACF Image field - Possible to cross-refence the fieldname to the acf-json/* files?
                    if (false) {
                      if (str_starts_with($fieldname, 'image')) {
                        $block['acf_image'] = get_field( $block['attrs']['data'][$fieldname] );
                        $block['attrs']['data'][$fieldname] = hatch_get_image($value);
                      }
                    }
                  }
                }
              }
              return $blocks;
            },
          ]
        );
      }
    }

    // If we want to have a flag existence of blocks like that other plugin...
    // register_rest_field($post_type, 'has_blocks', ['get_callback' => function (array $post) {
    // 	$blocks = parse_blocks($post['content']['raw']);
    // 	$blocks = array_filter($blocks, function ($var) {
    // 		return !empty($var['blockName']);
    // 	});
    // 	return count($blocks) > 0;
    // }]);

    // Featured images for pages and posts
    add_action('rest_api_init', function () {
      register_rest_field('pages', 'featured_image_src', array(
        'get_callback' => function ($post_arr) {
          $image_src_arr = wp_get_attachment_image_src($post_arr['featured_media'], 'medium');

          return $image_src_arr[0];
        },
        'update_callback' => null,
        'schema' => null
      ));
    });
    add_action('rest_api_init', function () {
      register_rest_field('post', 'featured_image_src', array(
        'get_callback' => function ($post_arr) {
          $image_src_arr = wp_get_attachment_image_src($post_arr['featured_media'], 'medium');

          return $image_src_arr[0];
        },
        'update_callback' => null,
        'schema' => null
      ));
    });
  }
);