Here, you can see various methods for authenticating with the indexer:
OpenSearch Authentication Documentation. Currently, you are utilizing basic authentication, but you have the option to switch to JWT (JSON Web Token) authentication. With JWT, you can employ the "Authorization: Bearer" header for authentication.
Additionally, another option you can explore is configuring Filebeat to read the JSON file you mentioned, in addition to the standard "alerts.json" file (or with a second Filebeat). This allows you to ingest data from both sources seamlessly.
I hope it helps.