If you want to continue to use a walking foot with feed dogs down, I would recommend the book Walk by Jacquie Gehring. Her instructions really open up additional possibilities for quilting with a walking foot. Definitely beyond stitching just in a straight line.
If you want to learn free motion quilting, which drops the feed dogs and uses a darning foot or similar, I would highly recommend the Free Motion Quilting Academy taught online by HollyAnne Knight of String and Story. Amazing class designed to give you confidence to quilt on your DSM or a long arm. I have attached a few of the motifs I did on the sampler we did in her class.