When you cut your piece, do you only cut the 3mm steps down, and don't do a finish pass??

I would after you have done your rough cut, do a finish pass, which should (theoretically) make the lines disappear. But it's not for certain. You might have tool deflection.

Have you checked up on the fact that you might have deflection of the gantry itself? It looks quite light, and not very sturdy. That could be your problem.

And last but not least, check up on your tool. It might not be the size you think it is. Some of those cheap chinese cutting tools are not what they should be.