SKU Offer for each variation

Hello,

I am using the following code to otpimize my structured data. I use offer since all of my products are variable products.

// GTIN données structurées

add_filter( 'rank_math/snippet/rich_snippet_product_entity', function( $entity ) {
    if ( ! is_product() ) {
        return $entity;
    }
    
    $product = wc_get_product( get_the_ID() );
    if ( ! $product->is_type( 'variable' ) ) {
        $entity['gtin13'] = get_post_meta( get_the_ID(), 'hwp_var_gtin', true );
        return $entity;
    }
    $variations = $product->get_available_variations();
    
    if ( ! empty( $variations ) ) {
        $entity['gtin13'] = get_post_meta( $variations[0]['variation_id'], 'hwp_var_gtin', true );
    }

    return $entity;
} );

add_filter( 'rank_math/snippet/rich_snippet_product_entity', function( $entity ) {
	if ( ! is_product() ) {
		return $entity;
	}

	$product = wc_get_product( get_the_ID() );
	if ( ! $product->is_type( 'variable' ) ) {
		$entity['gtin13'] = get_post_meta( get_the_ID(), 'hwp_var_gtin', true );
		return $entity;
	}

	$variations = $product->get_available_variations();
	if ( ! empty( $variations ) ) {
		$entity['offers'] = [];
		foreach ( $variations as $variation ) {
			$price_valid_until = get_post_meta( $variation['variation_id'], '_sale_price_dates_to', true );
			$entity['offers'][] = [
				'@type'           => 'Offer',
				'description'     => strip_tags( $variation['variation_description'] ),
				'price'           => $variation['display_price'],
				'priceCurrency'   => get_woocommerce_currency(),
				'availability'    => $variation['is_in_stock'] ? 'https://schema.org/InStock' : 'https://schema.org/OutOfStock',
				'itemCondition'   => 'NewCondition',
				'priceValidUntil' => $price_valid_until ? date_i18n( 'Y-m-d', $price_valid_until ) : '2025-12-31',
				'url'             => $product->get_permalink(),
				'gtin13'          => get_post_meta($variation['variation_id'],'hwp_var_gtin', true ),
			];
		}
	}

	return $entity;
} );

As you can see by testing on this url: https://lerepaireduchef.fr/barbecue-kamado/

I do have a GTIN code for each offer and therefore each variation. Likewise, I would like to have a SKU code for each of the offers and variations. Everything is already well filled in woocommerce.

Thanks in advance