Another thing I noticed the shared_buffer parameters set as 2029684 in this instance, which comes to ~21MB and that seems very small for a database operating in large scale. And I see we have RAM in the instance showing as ~256GB. So thinking of bumping it to something as ~40-50GB.
shared_buffers has a unit of 8 kb blocks, so your cluster is set at 15GB, not 21 MB. Even so, going to 50 would be fine if you have that much RAM.
Hope that will help to some extent. Not sure if there is methods to manually, cache some objects(tables/indexes) which were getting used frequently by the read queries.
That's one of the points of shared_buffers - keep things that are accessed often in memory. Postgres keeps track of which things are used more often, so in theory the most frequently used items are removed only when absolutely necessary.