When we think about best UY, we look at when we travel most and are most likely to make a reservation. You list Aug-Dec. Then we look at any UY minus 4 months, so we could still have an option to bank points should a reservation need to be cancelled. Since your travel would be complete by Jan 1, possibly, you'd still want to be able to bank points (at least have the option, should a trip fall through). If you have a June UY, you can still bank points until the end of Jan. If you have an Aug UY, you can bank until the end of March. So I think your 'summer' month options would make sense - if you buy into the logic we used! Not sure if this helps and makes it worse! Just our opinion!
I doubt that UY will impact the cost of a resale contract - that rarely seems to be a factor, at least not that we've seen. Good luck with your purchase! Could be a really good time to buy.