Boy, that is a tough one.
Unfortunately it does seem to be getting harder and harder to predict crowd patterns.
What with year round schools, floating breaks, cheerleading competitions, conventions, discount promotions, etc. etc. even if you go at a time that has historically been slow, it could very well end up being downright ugly crowd-wise. I know.
We have gone every Oct. for years and although I think the days of walk ons for everything are gone, it's still usually very nice. Last year it was an absolute zoo. We scrapped our Oct trip this year for the 1st time in ages.
How about picking a time that is likely to be great weather-wise, going mid-week and taking your best shot at low crowds?
I know when we take my elderly in-laws there are not all that many things that are must dos for them, so even if it's more crowded than we like, we just drop the commando mode, slow things way down and make sure we get to the things they really enjoy.
I hope you can make it work.