Sorry to hear that. When the K2 works it works well. not getting past the slicer really points to a model issue. Have you tried an alternate slicer? An easy quick check would be to use creaility’s cloud slice. I’m really curious about the model that won’t slice. Did you find anything in the gcode? If you can think of a way to get it to me I’ll give it a try.
Separately, here’s another person with CFS issues similar to yours. You are not alone on CFS issues. K2 plus CFS always prints with first spool no matter how sliced or chosen