Access denied in OpenSearch Serverless

Question

I am trying to create a minimal working example for working with AWS OpenSearch Serverless. With the help of this tutorial, this is the code:

import boto3
from opensearchpy import OpenSearch, RequestsHttpConnection, AWSV4SignerAuth

host = 'onb565zzbfkjr3spn8v5.us-east-1.aoss.amazonaws.com'
region = 'us-east-1'

credentials = boto3.Session().get_credentials()
auth = AWSV4SignerAuth(credentials, region)
client = OpenSearch(
    hosts = [{
        'host': host,
        'port': 443
    }],
    http_auth = auth,
    use_ssl = True,
    verify_certs=True,
    connection_class = RequestsHttpConnection
)

def create_index(index_name):
    index_body = {
      'settings': {
        'index': {
          'number_of_shards': 1
        }
      }
    }
    response = client.indices.create(index_name, body=index_body)
    print('\nCreating index:')
    print(response)

create_index('myindex')

I have performed the following steps:

1. Created an IAM user that has the policies AmazonOpenSearchServiceFullAccess and AmazonESFullAccess (just in case). I also added two inline policies:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "aoss:APIAccessAll",
            "Resource": "*"
        }
    ]
}

and

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "aoss:DashboardsAccessAll",
            "Resource": "*"
        }
    ]
}

(for some reason, the latter two permissions are not shown when I create a collection)

2. Executed aws configure to provide the keys and the region.

3. Created a collection with the rule for Public access, the IAM user as the selected principal, and all accesses enabled.

Despite all this, I get 403 (Access denied) when trying to create an index. What could I be missing?

UPDATE I have now asked the same question in the AWS community.