We're going slightly earlier than you - leaving a couple days after your trip starts so I've been keeping an eye on things. The crowds should be at some of the lowest. (We went one year at the end of January during a record cold winter though and I wouldn't be surprised if that was really the lowest though). We normally go Nov/Dec non-holiday and the crowds are quite managable then. From what I've read, the crowds are lower than that in Sept so I know we can handle them. (I'm not so sure about the heat though, but we'll be trying!)
About Epcot F&W, it does increase crowds in EP significantly. However, our lowest Epcot day ever was the last Sat of F&W! The reason why it was so low crowd-wise was due to how/what we did. We had more than one EP day, so that day, we were there at rope drop and focused on Future World. We did make it over to World Showcase but barely, and then left - doing the rest of World Showcase on a different day. So, if your plan is to come early and not to stay late and you're going to focus on Future World, then it can be quite managable. But if your goal is to enjoy F&W to the fullest, then expect large crowds.
Even with low crowds, expect some rides to be crowded. We've found Space Mountain can be a walk-on during many Nov/Dec trips - so really low crowds. But TSM would still have waits of over an hour starting within an hour of opening. Use your FP+ wisely to do those rides and/or hit them first thing. (Though even that may not always work - for example, Anna & Elsa is not something I'd suggest attempting without a FP+ even if you're there at rope drop).
Another thing to consider is free dining. I didn't check your dates, but our Sept trip is during free dining. Free dining can make the parks feel a lot more crowded, but people eating longer meals can help with ride line length. I've found parks to feel a lot more crowded than the ride times showed during free dining in the past. We found AK to be the worse park for feeling crowded even when ride times were fine.
