Stresses in a clamped circular plate
At the beginning of Chapter 10 in Roark's, it states that the flat plate formulas apply to thicknesses "not more than about one quarter of the least traverse dimension". In this case your least traverse dimension is 6 times your thickness, so you should be able to use the formulas.
As for the deflection issue and non-uniform loading, with 600kN on this thing (67 tons!!) I'm curious what is causing this loading and what material you're going to use to resist it...