Thanos is probably one of the other popular choices. It's being heavily used in production by a number of companies, but I don't think they've branded it at "Prod ready" in a 1.0 release though.
Interestingly, it looks like Cortex's support for local storage and object stores comes from using Thanos's storage engine. So once it's production ready in Thanos it will probably be production-ready in Cortex shortly thereafter.
https://cortexmetrics.io/docs/operations/blocks-storage/
I think for Cortex your safest storage options now are Bigtable, DynamoDB, or Cassandra.
What you said is correct and makes sense. Though, I would suspect either choice works with any S3 compatible API that can run on local storage, but I know that isn't necessarily what's meant by "local storage".
Batteries included.