Nowhere near 100GB/day so I will drop the 5 shards down to... 2? I will have to understand that better before I make a decision. Happy to listen to any suggestion you may have.
There are some good chapters in Elasticsearch: The Definitive Guide that talk about scaling, shard sizees, etc. It regularly comes up here too. I don't have the whole picture but I suspect a single shard will be fine in your case.