Your quilt is beautiful! Absolutely beautiful!
I'm very much a beginner, and only sharing what I learned about those wavy borders after a quilting retreat in Texas last fall hosted by The Quilt Mercantile in Celeste, Texas. It worked beautifully on the quilt that I was making that weekend (which was also my first!)
The teacher's name has slipped my mind, and I am so sorry about that... she was great!
She used binding cut on the straight grain, not bias as that automatically gives you stretch! Using the binding strips, measure through the center line of the quilt for the lengths needed. Both directions, of course, for a total binding length. (Not along the edge and not using a measuring tape. Less margin of error!) Divide the binding in half or quarters, depending on the size of your quilt. You just need manageable sections. Mark and pin the corners and halfway points, then pin the binding all the way around, easing in any extra in each section. It's much easier to ease in small sections. Then use a walking foot to attach the binding. It will absolutely lay flat!
If you live close to Celeste, which is in northeast Texas close to Sherman, you must visit this shop! They are the best and have awesome retreats in the spring and fall. Unfortunately, I don't live close enough to take their classes! And, again, my apologies to that wonderful teacher! Old age is a terrible thing...
Sara