Optimization: Increase Innodb_pool_size useful?
It's good you don't see a lot of reads, ideally it should be zero if available memory permits.

When InnoDB needs a page it checks the buffer pool. If the page is in the buffer pool - done. If not - InnoDB will read it from the disk and cache it in the buffer pool. So, if the data set fits into the buffer pool there will be eventually no reads from disk.

If you already get no reads further buffer pool increase won't increase performance.

The storage drop - I don't know RDS is a black box, who know what's going on there.