I have a DII, which probably isn't much like the Sapphire, but my satin stitch is very smooth and velvety-looking. I use 60wt cotton thread, top and bottom, or else the Aurifil 50 on top, and have a bobbin case with the tension set for it. I use a 70/10 needle, so it doesn't make enormous holes. Once you get past the uneven length thing, which seems like a repair issue, maybe going fine for the thread and needle will be helpful.
hugs,
Charlotte