Your mother has a talent! Those are stunning!!
To show off a center I prefer a star-burst design or a cross hatch rather than echo quilting. To me, echoing detracts from the design if more than a single or double echo.
Perhaps "frame" each one in a heart or diamond shape with star burst straight lines, then between each shape either cross-hatch, stiple, or some other simple filler that is more dense to make the shape with the embroidery pop out.