Have you done any test carves yet?
I ask because I was in a very similar spot as you with MDF spoilboards on my MPCNC and I was asking for advice on surfacing. The response that I got was not likely to be needed and more of an advanced task.
I did the test crown and it turned out better than before I had the spoil board w/ marker.
Once you get the test crown carved, then you can see if you are having leveling issues.
Not a grid but I am using t track at a 3" spacing and have been pretty happy with that so far.