For your UY, you've identified the months you are MOST LIKELY to travel to WDW - September and mid-summer (I'll just say July and August for that). That is part of picking the best UY. To pick the best UY for you, you also need to consider the months you are LEAST LIKELY to travel to WDW. You want those months to come just before your UY begins.
So, if you think you will almost never travel during November and December, then I'd probably look at a February UY.
If you think you might want to travel around the holidays or the winter months, but you're definitely not going in late spring (April/May), then you might want to look for a June UY.
What you want to avoid is (1) ever having to cancel within 30 days of a reservation no matter your UY, or (2) canceling a reservation in the last 3 months of your UY, but even with 30+ days notice. If you do #1, no matter when, those points go into "holding." If you do #2, they won't go into holding, but you will only have to the end of that UY to use them or lose them which can be as bad or worse than points going into holding.