Did you mean to attach a drawing to this post? It's hard to do more than hazard wild guesses unless we know how you plan to use these various components and materials. Was the concrete in place of the epoxy-granite or an alternative? Which hybrid steppers did you buy? How fast did the manufacturer say they'd go? Holding torque is not a good indicator of performance, but it's rare to see steppers geared up for speed. What do you mean by "having the 4th axis in the y-axis upright"? That spindle sounds like it might work for aluminum and softer materials, but not for machining steel except for with very small cutters.