You can have individual methods in a repository executed asynchronously, regardless of whether you have passed your own ExecutorService in the MongoManager or not.
This works via the @Async annotation, via the desired method in the Repository. However, the method only returns a CompletableFuture with its value instead of the value directly.
The only thing that changes in the method declaration is the return value, which wraps the previous one in a CompletableFuture.