I wanted to suggest something--but you already have sewn the pieced border on. I thought if you were going to have other borders on each side of this pieced one, you could first sew
borders on the strip piecing, then treat this whole unit as
one border. easier to handle a stretchy border if it is "held down" by outside borders. then you can miter or square
the corners or put cornerstones in. I do a lot of my borders like this so you only have to sew onto the quilt once.