I had this problem on the quilt I just finished, it had those same bias triangles all around the edge. The middle looked fine. I ended up not putting a border around it and it is just a little bit ruffly. I tried to not stretch the bias edges when I put the binding on, but it still shows just a bit. Next time I use bias triangles, I'm going to be a lot more careful. I've learned tons since I started. Measuring the center would have solved it if I had added a border.
But, the recipient love it, so no problem!
I love this pattern and the colors, and if it were going to be my quilt, I wouldn't mind the not-flat thing. You still have an outstanding quilt on your hands! Good for you!