I never stitch in the ditch because I'm BAD at it. For straight line quilting, I'm a fan of echoing or cross hatching. I don't have a STD foot so I can't speak of that. A walking foot is nice when doing anything thick. It helps when sewing on bindings or sewing heavy duty fabrics.
I feel confident that your quilt does not need any additional quilting added. It sounds more heavily quilted than many that have survived years around here. Of course, if it makes you feel better, add more quilting.
Some of the best advice on the quilting process I've heard came from Holice here on the board. (He is a famous quilter in case you don't know.) Holice says you need to be thinking about how the quilt will be quilted as you make the quilt. I used to just make a quilt and then try to figure it out. Now that I plan it as part of the quilt, it is much easier.