Here is another thought.
If you are at the outside edges of the quilt, do you really need QAYG technique? You no longer have to attach more blocks. You are finished with QAYG.
Miter the front border fabrics in the corner.
Miter the back border fabrics n the corner.
Miter tape the batting corners.
Smooth the layers and quilt.
If you want to miter multiple borders, you sew all the borders together for one border and miter them all at one time not individually.
In my mind, I can see this working. Am I missing something?