The feed dogs are not the reason for the problem you are having. You need to adjust your tension. Start with the standard tension. If your machine works fine sewing a straight line with the regular foot, switch to the darning foot and try again without changing the tension. The secret here is to sew very fast but move the fabric slowly. You will learn to achieve even stitches, but while you are learning they may not all be the same size. Is OK. It takes time. If at the same tension it doesn't work, adjust your tension little bits at a time. Make sure you have a nice sharp needle and good quality thread. With your machine I wouldn't try anything smaller than 40 wt. Fine threads don't work well on all machines.
Also, I may get in trouble for saying this, not every machine works well for FMQ. I can say this from experience. I had a Kenmore and no matter what I did I got eyelashes on the back. I had 3 different feet and sometimes I couldn't do it with one, switch feet and the other one would work well for a little bit. It was completely unpredictable. When I switched to an older Bernina I can say that I achieved good FMQ. What I am trying to say is not to put all the pressure on something you may be doing wrong. It may just be your machine.
Keep trying but don't change more thna one thing at a time, otherwise you will never know what fixed the problem.