Yes the weather is much more humid and hot in august than in May and December usually isn't hot or humid.
Also be advised that they have a BB offer going right for free dining next summer through Sept. 7 so this may have some impact on availability of the GP free dining when it is released. Also don't know where you are planning on staying on site but some resorts are blocked out for free dining.
Also some schools may be back in session on your August dates but many will not of started yet. But all schools would be back in session by your Sept. dates. So for crowds I would vote for Sept. for weather I would vote for Oct. for FD I would vote for Aug. or Sept.
I've always wanted to do a Sept. Trip for my birthday but haven't yet because I hate being drenched by rain/sweat lol. Maybe one year I will just go for it and see what happens.
The first party is usually always crowded MNSSHP/
MVMCP. But so far there are reports saying the second one was overly crowded also but that was on a Friday night so probably a lot of locals also. I'm waiting to see how the Sept. crowds are as I think they will be some of the least crowded this year.
We are planning a big family trip for Oct. 2019 and doing MNSSHP but I am already planning on it being really crowded and busy. Not my ideal time to go but that's the time everyone's work schedules could accommodate vacation time. I really wanted to go early Dec. and do MVMCP but 2 of my adult children are blocked from taking any time off during that month. So Halloween it is lol. And it is Food and Wine so that increases crowds in Oct.