You could increase the number of primary shards for your index, which would allow you 2^31 documents per primary shard.
You could goto a weekly index ~4 times the amount of shards you currently have per month
You could goto a daily index ~30 times the amount of shards you currently have per month
Which is best is going to be based on many factors, including but not limited to:
Your cluster architecture, number of nodes, indexing pressure, read pressure, number of cpus available, amount of heap and ram available, speed of disks... The best approach would be to test, test, and test again.
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