Gutenberg lazy loading content

Hi,
I’m trying to add a “lazy” field to the “content” endpoint using “rest_prepare_post”.

I do this by adding “rest_prepare_post” filter.
Then I check if “$post->post_content” has gutenberg blocks.
After that I search in the loop all those that have blockName set as “core/image”.
Then I edit the contents of this block.

My problem is to put a changed HTML into “content” field, which will be a copy of “content[rendered]”.
Below I insert the code.

class EndpointLazyLoading
{
    public function __construct()
    {
        add_filter( 'rest_prepare_post', [ $this, 'add_content_lazy_field' ], 1, 3);
    }

    public function add_content_lazy_field($response, $post, $request)
    {
        if (has_blocks($post->post_content)) {
            $blocks = parse_blocks($post->post_content);

            foreach ($blocks as &$block) {
                if (isset($block['blockName']) && $block['blockName'] === 'core/image') {
                    // some magic with $block['innerContent'][0] and $block['innerHTML']
                }
            }
            unset($bloc);

            $response->data['content']['lazy'] = [
                'markup'  => post_password_required($post) 
                    ? '' 
                    : '^^^^^^^^content that i try to add^^^^^^^^',
                'protected' => (bool) $post->post_password
            ];
        }
    }
}

I want to see something like this:

GET domain.com/blog/wp-json/wp/v2/posts/6222?_lazy
{
	"id": 1,
        (...),
	"content": {
		"rendered": "original content",
		"protected": false,
		"lazy": {
			"markup": "original content with changed image block (wp:image block)"
		}
	},
        (...)
}