Park Suggestions based on Recent Crowd Underestimates? Are available Fast Passes reliable to judge crowds?

Hello - Arriving at Disney World 9/25. Have 4-Day Park Hopper to use between 9/26-10/3. Any suggestions what days to go to the parks ? Touring Plans seems to have been underestimating crowds, and since we have the flexibility we would like to hit the lowest crowds. We will arrive before opening each day.

Looking at the FastPass availability tool, it seems like some days are better than others - is that reliable?Thanks!

Are you looking here?

It does look pretty good on many days!