Best way to access AWS RDS from AWS ECS Fargate

I have Java based containers running AWS ECS Fargate. Now we are accessing AWS RDS Postgres database via username-password combo, but I wonder how to make this process more secure.

But I have a concern: How to avoid storing user passwords in .psql scripts (CREATE USER username WITH PASSWORD 'xxx')? I think better option will be to avoid using passwords completely, but I am not sure how to do it...

Loading passwords to Fargate containers is from AWS Secrets Manager - this is OK.

Do you have any advice, please?

EDIT: I will share my progress here so anyone can reproduce
Docs: https://docs.aws.amazon.com/.../.../.../UsingWithRDS.IAMDBAuth.html

- PHASE ONE -
Goal: PoC of IAM Auth (using IAM User)

1. I have prepared my RDS instance to allow IAM Auth
2. I have created IAM user, attached new IAM Policy to it and create an access key.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "rds-db:*",
            "Resource": "arn:aws:rds-db:eu-central-1:666:dbuser:db-YFVS/iam_user"
        }
    ]
}

NOTE: In the resource is used resourceId, not arn! (this can be found in the Configuration tab of the rds instance)

3. I have created user in my database

CREATE USER iam_user; 
GRANT rds_iam TO iam_user;

4. I tested the connection (after exporting access key)

# Generate token
export RDSHOST="dev.aaaaa.eu-central-1.rds.amazonaws.com"
export PGPASSWORD="$(aws rds generate-db-auth-token --hostname $RDSHOST --port 5432 --region eu-central-1 --username iam_user )"

#Connect
psql "host=$RDSHOST port=5432 dbname=mydb user=iam_user password=$PGPASSWORD"

mydb->

Now I would like to proceed with Java implementation and furthermore move to Fargate

- PHASE TWO -
Goal: Implement IAM Auth into Python app I have followed the tutorial here and it worked fine. I have used IAM credentials (key, secret) of the IAM user I created in the previous step. Also, I have loaded creds from ENVs instead of profile file.

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.IAMDBAuth.Connecting.Python.html

import psycopg2
import sys
import boto3

ENDPOINT="postgresmydb.123456789012.us-east-1.rds.amazonaws.com"
PORT="5432"
USR="jane_doe"
REGION="us-east-1"
DBNAME="mydb"

session = boto3.Session(region_name='us-east-1',
                        aws_access_key_id=os.environ['AWS_ACCESS_KEY_ID'],
                        aws_secret_access_key=os.environ['AWS_SECRET_ACCESS_KEY'])

token = client.generate_db_auth_token(DBHostname=ENDPOINT, Port=PORT, DBUsername=USR, Region=REGION)

try:
    conn = psycopg2.connect(host=ENDPOINT, port=PORT, database=DBNAME, user=USR, password=token)
    cur = conn.cursor()
    cur.execute("""SELECT now()""")
    query_results = cur.fetchall()
    print(query_results)
except Exception as e:
    print("Database connection failed due to {}".format(e))