Track purchases by SKU in Google Analytics -

our products apparel, have lots of sizes , color variants roll under single product id.

we have google tag manager integrated google analytics , enhanced ecommerce. on product detail page, product id (parent level, no variants) pushed datalayer in sku variable. on confirmation page, datalayer > transactionproducts populated array of each sku ordered. sku @ parent level. hence, can see product page conversion rate based on sku ordered vs sku passed product impression.

google analytics take first instance of sku value , ignore rest. so, in below example, google analytics show 3 units of sku 123 sold in order.

datalayer = [{     "pagename":"confirmation",     "pagecategory":"checkout",     "transactionproducts": [{         "sku":"123",         "vn":"123:black",         "price":10,         "quantity":3,         "currency":"usd"     },{         "sku":"123",         "vn":"123:white",         "price":10,         "quantity":2,         "currency":"usd"     }] }]; 

my question this: how can track purchases in example above such total quantity of sku 123 recorded in ga?

by lights, either 1. change sku means on product page vs confirmation page, , keep sku parent in former , update sku vn on latter or 2. give ability see purchases of variants , push parent-level sum totals transactionproducts in datalayer on confirmation page. if 1, fear we'll loose product page impression conversion rate metric. if 2, loose visibility purchases variant. there other option tracking total sku's purchased missing?