$objectManager = MagentoFrameworkAppObjectManager::getInstance();
$product = $objectManager->create('MagentoCatalogModelProduct')->load($_item->getProduct()->getId());
$imagewidth=30;
$imageheight=30;
//$_product = $objectManager->get('MagentoCatalogModelProduct')->load($_item->getProductId());
$imageHelper  = $objectManager->get('MagentoCatalogHelperImage');
$image_url = $imageHelper->init($product, 'product_page_image_small')->setImageFile($product->getFile())->resize($imagewidth, $imageheight)->getUrl();

    <div class="product-name"><a href="<?= $_item->getProduct()->getProductUrl(); ?>"> <?= $block->escapeHtml($_item->getName()) ?></a></div>
    <div class="sku"><?= /* @escapeNotVerified */  __('SKU'); ?>: <?= $block->escapeHtml($block->getSku($_item)) ?></div>
    <div class="sku"><a href="<?= $_item->getProduct()->getProductUrl(); ?>"> <img src="<?= $image_url?>"></a></div>

Thank you{{config path="trans_email/ident_custom1/name"}} for your order.

{{block class="MagentoFrameworkViewElementTemplate" order=$order name="order_sms" template="module_sales::items.phtml" area="frontend"}}