I have alot of expierence grinding D2. Horrible stuff it is...
if you are doing 6x6 and .003/pass, Im guessing you have a fairly large surface grinder running flood coolant.
D2 will load a wheel faster than you can blink.
I dont think that you can get those plates anywhere near flat with a simple on/off electric or manual magnetic chuck.
If the D2 comes to you for grinding, in a hardened state, look at it on a surface plate with an indicator running over it. It will obviously, be way out of flat. This is the result of the heat treating, and the relative thinness of the part.
The ONLY way to bring a "pringle" shaped piece of hardened metal into "flat" is to either grind it while being held in a mechanical (non magnetic) holder like epoxy or clay (I do not recommend this) or, the right way is to have a well "dressed" variable strength magnetic chuck holding the part at only the strength required to keep the part on the chuck. For safety reasons, you will need to "block in" the part you are grinding. this is a way that you can use minimal magnetic force. minimal magnetic force allows you to let the part be in "pringle" form and take down the high spots, flip, take down the high spots on the other side and repeat ad-neausum until you have brought the part into "flat"
I used to frequently grind D2 to a micron or less (without coolant) on various parts using the above method. I was trained from scratch as a Grinder Hand, and I worked at a shop that did world class grinding. If their were another way to get a thin part flat to a micron or less, we would have figured it out. At least if involved grinding.
I can say with authority, that without a variable strength chuck, you will be chaseing flat forever.
Frankly, grinding the parts you are talking about is one of the most difficult grinding jobs I can think of. Anyone with a half assed machine and a bit of training can grind a solid, stable thick part fairly flat. What you are doing is not easy or slow.
I wish I had more encouraging things to say, but I suspect, its time for a new chuck.