You can’t bank point until you enter the UY of the points. So, August 2020 UY points can be banked, starting on August 1st, 2020. Once banked, they will be eligible for booking trips in your 2021 UY.
The easiest way to know what points you can use for trips, forget about calendar year and when you will be calling to book trip. Those don’t matter, and just focus on UY of the travel.
Any trip can be paid for with banked points from UY prior to the trip, points from the UY of the trip, and borrowed points from UY after the trip that are needed to complete the reservstion,
So, as mentioned, since your Dec 2021 trip will be in your 2021 UY, you can use banked points from 2020, 2021 points, and borrow 2022 points.
As long as you bank your 2020 points prior to your 11 month window..January 2021..they will be available for use when you book.