Hi, I don't want to discourage your attempt at using piping.... that said, piping on a quilt is usually small, with a very thin cord in it. Susan Cleveland has a book and ruler out that are great.
If it were mine.... I would audition a small back border to replace the solid blue. If I had enough of the sashing fabric I would try that for the outer border. This would leave some room for curvy quilting that would balance the geometric pieced center. This is only what I would do... it's your quilt but you did ask for opinions