Elasticsearch + Clustering
I'm afraid I have zero experience with ElasticSearch, so I can't help with that but here are a few thoughts about the process you propose.
Overall I think the approach makes sense, especially in terms of efficiency but also check the following points:

This is not strictly speaking a clustering task, in the sense that the system doesn't find itself how to separate the clusters since this is based on a predefined threshold.
The threshold is crucial and you need a method to determine it. You might also want to plan for updating the threshold as the collection grows. The process is likely to lead to some very big clusters and many very small ones (unique document in a cluster).
If you assign a document to the best match cluster, it means that a document belongs to a single cluster. Depending on the application this could be problematic, with some documents expected in a cluster but found in another.