Traditionally, mid-september is not too crowded because it still has summer weather, plus increased likelihood of hurricanes, plus is right after school starts but before fall breaks begin.
However, all bets are off this year because there are many who will want to avoid the 50th anniversary crowds that begin October, or see HEA one last time, etc., and people are comfortable traveling to theme parks more now that they are vaccinated. Plus, Disney's seasonal pricing actually help distribute crowds more throughout the year--it is not just the tickets, rooms are cheaper too (both cash rooms and DVC rooms rented/booked on points). So you get bargain hunters then too. The other thing to consider is that Disney often cuts back on staffing when parks aren't as busy, so that can keep wait times higher even though crowds are shorter (e.g., rides like space or big thunder that normally have two loading areas will only have one running) .
I think crowds THIS september will be lower than they are currently, and lower than Oct when the anniversary happens, but higher than most septembers.