Easiest way to figure what can be used. First, determine UY of the trip. Trips can be booked with banked points from the UY prior to the trip, points from the UY of the trip, and borrowed points from the UY after the trip.
So, with an April UY, summer 2020 falls in your 2020 UY...so when you call and book, you could use banked 2019 UY points (if you had them), 2020 UY points (the 6 you have), and borrow from 2021 UY (up to what you still have).