I would think a September or October UY would be ideal for your preferred travel.
A September UY would cover your Sep/Oct F&W travel and December travel. Your banking deadline would be Apr 30, but I still don't see that being problematic for your May travel. If you did have to cancel your May trip, you'd want to cancel at least 31 days out anyway so your points don't go into holding (which is what happens for trips cancelled 1-30 days prior to your check-in date) and you'd still be prior to your banking deadline of Apr 30 if cancelling a May trip 31 days out. If you did have to cancel 30 days or less than you'd have all of June, July and August still to use/rent out those points.
An October UY wouldn't be helpful for as far as flexibility for Sep F&W travel, but you would be good for October F&W travel, December travel, and May travel with a banking deadline of May 31. I suppose if you had any travel issues, had to cancel a reservation and end up with holding points, you could use those holding points for Sep F&W travel.