If you don't want to worry about database/server administration and scaling issues, I'd recommend looking at Cloudant. It's quite inexpensive, takes virtually no administration effort, scales automatically, and is fast and flexible.

You could continue to use Redis as a write cache, although a message queue or something like Kafka might be better for that purpose, and of course as a read cache, while using Cloudant or some other "real" database for permanent storage.

(On a side note, you described your application as "write-heavy" but you also said you get "about 3 Million individual customer orders on monthly basis" — which works out to just slightly more than one record to insert per second, on average, which doesn't seem "write-heavy" to me. I understand that you get orders submitted in batches, rather than evenly distributed, but it still doesn't seem especially write heavy. At any rate, a database like Cloudant can take as many writes as you want to throw at it.)

(Disclaimer: I don't work for Cloudant; I am a satisfied customer; I don't use Cloudant for everything.)