Protein extraction definitely sounds like it could be a problem.
I have not used a kitchen aid, but I’ve used another style of stand mixer, and I think it does a good job of mixing small batches of meat. The Weston meat mixers will work better if there is a slightly larger batch size.
I would assume the kitchen aids have enough power to mix the meat fine, but maybe someone else has tried it and could chime in for confirmation?
If your sticks are fatting out, the other possible problem could be too quick of a temperature rise, too high of heat, and too fast of cooking. Depends on what your cooking schedule was like.
Carrot fiber is not going to help with binding from a protein extraction perspective. It will help bind and hold water though. You can add a different binder like Sure Gel or Soy Protein Blend, which will have a better effect on helping increase protein extraction during mixing. Either of those two will slightly increase your protein content, and the more protein in the meat, the easier it is to get protein extraction and the protein extraction is what helps bind the fat in the meat. To quickly compare the 3 binder options, Sure Gel and Soy Protein are better for added protein and protein extraction (I’d choose Sure Gel over Soy Protein), but Carrot Fiber is probably better at water retention