Good news, I figured it out on my own! For keeping the variant title or id of the item updated on a ‘cart addition’, the trick is to add an iteration on product.variants within the ecommerce variable. Then point it to the suitable liquid variable – this could be variant.title or variant.id.

{% if template contains 'product' %}
 dataLayer.push({
   'ecommerce': null
 });
 var ecommerce = {
   'items': [{
     'item_id': {{product.id | json}},                 
     {% for variant in product.variants %}
     'item_variant': {{variant.title | json}},
     {% endfor %}               
     'item_name': {{product.title | json}},
     'price': {{product.price | divided_by: 100.0 | json}},
     'item_brand': {{product.vendor | json}},
     'item_category': {{product.type | json}},
     'item_list_name': {{collection.title | json}},
     'description': {{product.description | strip_newlines | strip_html | json}},
     'imageURL': 'https:{{product.featured_image.src|img_url:'grande'}}', 
     'productURL' : '{{product.url}}'
   }]
 };              
 $(__DL__.cartTriggers).click(function(){
   dataLayer.push({                    
     'event': 'add_to_cart',
     ecommerce
   });                  
 });              
{% endif %}

So there you have it! This code should take care of keeping the variant updated when adding to the cart.