I use A2 because it has the right combination of high hardness, machinability and is priced right for the application. If money were no object something like A11, 10V would be ideal.
Out of your list D2 has the best abrasion resistance, but its chip load requirements and cutting forces make milling that end radius across two inches and maintaining a flat smooth surface just about impossible and it can't be face milled to a high finish reliably because of its large carbides and high chromium content cause issues with BUE. But if you're not machining an end radius (used for plunge grinding) and if you're going to surface grind it, D2 would probably be the best. HRC 62-64 is a good hardness. Some amount of temper is necessary for dimensional stability, 400F is good. Being D2, you might need cryo to prevent RA from decomposing on the work surface over time causing the platen to warp convex with use. Of the steels you list, A2 is probably the most stable.
Be advised that even a hard platen will wear. The grinding grits we use cut hard steel no problem and they do find their way to the inside of the belt. Some belts have dry lube and cutting aids that build up black gunk on the platen and inside of the belt which accumulate these grits and can cut almost anything.
I do about 90% of my grinding with the top roller removed which puts a lot of wear on a platen, so I'm careful about avoiding conditions that would chew it up such as damp belts. I get about 100 knives on a hard platen before re flattening it, but my grinding technique relies on a very flat platen to work, otherwise problems in the platen are reflected in the work. If you're not grinding lengthwise that's less of an issue.
