GTIN numbers for variable products

We have a series of products each with variations in liters. For merchant centers, apparently we need to output individual offers within the Offers -> AggregateOffer attributes with @type: offer, price, priceCurrency, sku, name, availability and most importantly gtin13. I’ve seen variations of code to achieve parts of this equation but am struggling to output the proper format in a foreach loop. The only way I have found is to use the [$key] index entity. Unfortunately, this adds an index to the Offers -> AggregateOffers which of course is not a valid attribute:

“offers”: {
“0”: { // The property 0 is not recognized by Google for an object of type AggregateOffer.
“offer”: {
“@type”: “Offer”,
“name”: “1 Liter”,
“sku”: “1-liter”,
“gtin13”: “7424903198115”,
“price”: “19.29”,
“priceCurrency”: “EUR”,
“availability”: “InStock”
}
}, etc.

Without the [$key] I can only output the last variation. The rest of the code gathers the necessary fields properly. Here is the complete code I am using:

add_filter( 'rank_math/snippet/rich_snippet_product_entity', function( $entity ) {
    global $product;
    $entity['gtin8'] = $product->get_sku();
    if ( $product->is_type( 'variable' ) ) {
        $variations = $product->get_available_variations();
        foreach ( $variations as $key => $variation ) {
            foreach ($variation['attributes'] as $attribute => $term_slug ) {
                $taxonmomy = str_replace( 'attribute_', '', $attribute );
                $name   = get_term_by( 'slug', $term_slug, $taxonmomy )->name;
                $sku    = get_term_by( 'slug', $term_slug, $taxonmomy )->slug;
                $gtin   = get_post_meta( $variation['variation_id'], 'custom_gtin_field', true );
                $price  = $variation['display_regular_price'];
                $price  = number_format((float)$price, 2, '.', '');

                $entity['offers'][ $key ][ 'offer' ]['@type']           = 'Offer';
                $entity['offers'][ $key ][ 'offer' ]['name']            = $name;
                $entity['offers'][ $key ][ 'offer' ]['sku']             = $sku;
                $entity['offers'][ $key ][ 'offer' ]['gtin13']          = $gtin;
                $entity['offers'][ $key ][ 'offer' ]['price']           = $price;
                $entity['offers'][ $key ][ 'offer' ]['priceCurrency']   = "EUR";
                $entity['offers'][ $key ][ 'offer' ]['availability']    = "InStock";
            }
        }
    } else {
        $entity['gtin8'] = $product->get_sku();
    }
    return $entity;
});

As reference, here is one of the products on our site: https://www.ecoformeurope.com/product/cementmix-diy-make-waterproof-cement/

Hopefully you can help getting this to output properly.

Cheers,

Jason Jeffers