Trying to figure out
1 - How to control access to data (multi-tenancy where there are ~100K tenants)
2 - Database design - Indexes and Shards and best practices around mixing different types of documents in a single index.
Elasticsearch is built on-top-of Lucene which is a Java API that you can use in pretty much any application. If you already have a system that can search the MySQL clusters then I would recommend hooking Lucene into that system instead of standing up another one.
Choose one system and learn it well.