I went through the same thing you did last year and I tried everything you did. This year I went with a harder axle on the kart drives perfect and we no grip issues at all
granted we have a Tony Kart so we put in a hard axle and the kart has no binding or grip issues at all.
Im not sure which axle you would use to put in soft or hard as OTK karts are different than most. But basically what I’m saying is I had the same issue as you did the same setup changes as you and the. Finally I put a harder axle in and problem solved
maybe someone else can weigh in on if going to a different axle would help you as I don’t want to give you the wrong advice