The few times I've done it I used Aurifil 50wt colored thread on the top and Mettler 60wt (white) in the bobbin - painted directly on a quilt sandwiched with Warm & Natural batting, my painting was small - but dense. Did not have a problem.
Try the 60wt on a sample sandwich using your batting. It's possible that because it is so thin it just buries itself in the batting.
As for tearing the stitches out of what you already did ... I would say that after you try the 60wt thread on above sample sandwich - tear that out and see how that looks.