Hi Lisa, good to see you're still posting.
It was nice to meet you on Symi, even for such a short time - we had a ferry to catch!
I'm really pleased (though not surprised) to learn you enjoyed your visit and are planning to go back next year. Symi really does grab you, doesn't it?
To answer your 2019 question, I don't think you will notice any difference by going two weeks earlier. September is a popular month on Symi so the visitor numbers tend to be the same right up until early October. The weather isn't likely to be any different, it'll be lovely and warm, as will the sea.