Ok, now I have got a minimal version running on my local machine. What I did:

1. Making service_name configurable in order to set it to “aoss”
2. In AwsSigningRequestInterceptor.toAwsRequest I needed to add following line in order to get a valid signature

   awsRequestBuilder.appendHeader( "x-amz-content-sha256", "required" )

1. In ElasticsearchSchemaAccessor.waitForIndexStatus I skip the health check because _cluster/health is not supported for Serverless.
2. For the MassIndexer I had to skip the FlushWork and RefreshWork. Both _refresh and _flush endpoints return a 404.

After these modifications I got the minimal reproducer (see below) running.