That's a problem with AWS RDS PostgreSQL, not with Citus and TimescaleDB.

The Citus Cloud service is similar to RDS, but for Citus clusters. Every cluster runs on AWS in its own VPC and it lets you peer with the VPC. It also provides auto-failover, PITR, forking, read-only followers and scaling out without any downtime.

AWS RDS has tons of other problems. It's perfect for developement, but for production usage it might bite you.

The answer to that is to use Redshift which is a proper distributed column-oriented data warehouse. Partion on time and let the database do the rest.

Using standard Postgres with sharding for OLTP workloads is great but there are better options for OLAP, especially if you’re using managed services. Also there is the Citus cloud offering if you want to stay with Postgres.