If a store is doing that kind of volume, the question should always be about integration. What platform gives you the widest assortment of integration to scale - this includes predictive forecasting, supply-chain inventory management (re-orders from vendors based on sell rate), crm integration for customer support. I have customers on Shopify, and to my freaking surprise, they do 15+m a year. And the debate is always, "should we move away from Shopify? " Then we look at the integration because things like bad supply chain means the customer looses $50k a month in sales; leaving it on the table. Whether you like it or not, Shopify has very mature hooks into large data warehouses, BI, sales analytics, DSP omnichannel MARTECH/ADTECH systems. Personally, I would build it myself from scratch. But I'd have to build in all the ingress/egress data hooks.