Wny even use RDS? There is nothing relational about that schema. Just use DynamoDB. It'll be cheaper, you can actually query the JSON, and if you decide to flesh out the documents and go all in on NoSQL then you can do that easily.
It's PostgreSQL, so we can and do query the JSON data, and update deepely nested fields in the JSON natively as well (no need to take a lock, read the entire blob, modify it in the application, save the data, and release the lock).
PostgreSQL supports setting values, appending to arrays, etc etc natively in an ACID compliant way.
I'm not saying PostgreSQL is a bad choice by any means, or that DynamoDB is suitable for all projects. But good support for JSON is in no way a capability unique to PostgreSQL. At least ideally, a competently built document database should be at least as good if not better than an SQL database emulating a document database.
