Can you increase the size of the stitches? I find that when i do lettering, the heavier, density of the lettering stitches out nicer for me. I also put a light netting ( cant remember the name) on top of the fabric i will be stitching . You should be able to carefully pull the netting off the lettering. You may need to trim the netting with scissors too. It gives added support for the lettering stitches. And I agree, I use the same thread in the bobbin.

Lettering is tricky for me too.

Sandy