What a beautiful quilt. Love the layout.
In regards to your border you have received some excellent suggestions. I would only add to divide your quilt side and border both into quarters and then match the divisions so any easing that must be done will be done gently the full length of the quilt.
When I cut my borders I usually just lay the quilt out and put my border on top in the middle from side to side and then trim them to fit. Works a charm for me. No measuring tape necessary. But some people recommend that you do measure your quilt top in three places and then average them three measurements for the length of your border. Have I totally confused you yet?
Bottom line - just keep practicing. Your next quilt will have fewer waves, I'm sure!