Lovely blocks! I would wait until you have them all done to see if it needs a circle in the center square. One from the background fabric would look nice if needed.
I might do the dryer sheet or similar where you sew it to the right side of the fabric, trim with pinking shears and cut a slit in the dryer sheet to turn right side out.
You can also cut a cardboard template, run a gather stitch around the circle, gather it on the cardboard, iron and then remove the cardboard and applique down.