The two approaches that will require the least amount of development effort are configuring named credentials in the source org and using the Composite REST API to aggregate multiple leads in a single call.
Named credentials are a type of metadata that store authentication information for accessing external services, such as the target Salesforce org. By using named credentials, you can simplify the code for making callouts and avoid hardcoding credentials or tokens.
The Composite REST API is a resource that allows you to execute multiple REST API requests in a single call.
You can use the Composite REST API to create, update, or delete up to 25 records in one request.
This can reduce the number of API calls and improve performance.
References: [Named Credentials], [Composite Resources]

https://developer.salesforce.com/docs/atlas.en-us.platform_events.meta/platform_events/platform_event_limits.htm
https://help.salesforce.com/articleView?id=workflow_om_considerations.htm&type=5

Explanation
Using middleware queuing and buffering is a solution that can handle errors during service outages by storing the messages in a queue until the service is available again. This way, Salesforce does not lose any data or encounter any failures when the service is down. Using @future jobld and custom scheduled apex process to retry failed service calls is not a good solution because it can consume a lot of governor limits and create a lot of duplicate records. Using Outbound Messaging to automatically retry failed service calls is also not a good solution because it has a limited number of retries and a fixed retry interval, which may not be sufficient for long service outages. Using Platform Event replayld and custom scheduled Apex process to retrieve missed events is not applicable for this scenario because Platform Events are used for event-driven integration, not for web-service integration. Reference: Salesforce Integration Architecture Designer Resource Guide, page 29-30