historically trade in supply goes up after next gen announcements, driving the price down. https://www.gazelle.com/iphone/iphone-xr/t-mobile

might want to compare prices too first.

I would also add at this point, I’d be hard pressed to say there are going to be massive changes in the price you’d get. A phone tends to decline in value the most for the first few years it’s been released, sort of like how new cars depreciate the fastest in its first three years. So I suppose to answer your question: yes you’d maximize your money if you gave it up today, I just don’t know how much more you’d be making.