I think it would help to see your proportions and colors and such. If your blocks are small enough and square enough the actual "window" fabrics might not need any quilting.
I have an attic window top with fairly large (postcard sized/shaped) rectangles and it also has sashing. So far I'm planning on a scroll design in the black sashing, probably something linear and simple in the grey (so just echo the line 1/4" away from the ditch on both sides, and then we come to the rectangle...
I'm thinking a simple oval inside each postcard would hold things down and be sufficient quilting without distracting, while giving each block the same amount of quilting. I might connect with the diagonal lines, stitching in the ditch in the greys. I am trying for a specific look, think of the old scrap albums with the black pages.
https://www.quiltingboard.com/member...14-621650.html