Beautiful! Is it fused in place or just laid out? I think I might piece the background together if it is still loose. I too like the effect you have now without the black leading. If you want to add black leading, I would look for the prepared 1/4 inch bias leading. You could lightly zig zag down the sides without having to do a dense zig zag to cover the raw edges. You might need to add something behind the cross if the background is heavier with all the leading.
I would maybe use Hobbs 80/20 batt (extra layer behind the cross for a trapunto look) and single straight stitch down the black bias for quilting. Whatever you decide will be lovely!