There's a number of datastores that claim to shard and replicate automatically, with no worries for dev/ops/devops.

They've been lying. Never trust them.

Some datastores can actually do this, but performance per beefy server is less than you'd expect. You can use Riak but you have to write proper CRDTs. You can use zookeeper or etcd but those are for small amounts of configuration data, not for large amounts of customer data.

For all the datastores that claim to do everything automatically and have great performance, we can thank Aphyr for providing the proof that they don't live up to their promises, while we just suspect they don't.

I'd suggest trying to use a simpler model, and understand and accept its failure modes. Maybe your app has to go into read-only mode for a few hours if there's a server failure, etc.