if you use a lightweight fusable --- like steam a seam lite or heat & bond lite- when the finished project is laundered-it softens right up. the only ones that stay stiff are the heavier ones not meant to be sewn.- the ones used for wallhangings that are not intended to be laundered- or used as a quilt.
elmers glue and glue sticks wash out when laundered too. if you want it to be really soft while still working on it- try misty fuse-
also windowing means you trace your shape- then when you cut it out you also cut out the center of it- so only about 1/4" of it is adhered to the fabric- the center does not have fusable- it's only around the edges.