Magento 2 – How to Add CMS Block to Email Confirmation Email

add this to the your class:

private $blockFactory;

and make your constructor look like this:

public function __construct(
    PageFactory $pageFactory,
    \Magento\Cms\Model\BlockFactory $blockFactory
  )
{
    $this->pageFactory = $pageFactory;
    $this->blockFactory = $blockFactory;
}

you can even remove the $pageFactory dependency if you don't need it anymore.

Then replace:

$testPage = [
    'title' => 'Test page title',
    'identifier' => 'test-page',
    'stores' => [0],
    'is_active' => 1,
    'content_heading' => 'Test page heading',
    'content' => 'Test page content',
    'page_layout' => '1column'
];

$this->pageFactory->create()->setData($testPage)->save();

with

$testBlock = [
    'title' => 'Test block title',
    'identifier' => 'test-block',
    'stores' => [0],
    'is_active' => 1,
];
$this->blockFactory->create()->setData($testBlock)->save();

Your override file path is incorrect.

You have to just override order_new.html file at below location.

app/design/frontend/[Vendor_Name]/[Theme_Name]/Magento_Sales/email/order_new.html

Clear & Flush the Cache by using below command:-

sudo php bin/magento cache:clean
sudo php bin/magento cache:flush