The key here is wet season construction.
Lime is often used to aid the drying process. Usually 2 to 3 percent. With a fill height of 30 ft - not sure what the fill slopes are-, on ehas to be concerned about slope sloughs. On can try intermediate layers of gravel to aid in pore water presure dissipation of the fill is wet and no use of additives are required . Availability of granular material and cost comes in the picture as well. However,you noted that use of strong soils is not a solution.
The use of 6 inch fills while specified is hardly ever done or realistic. Try at least 500 mm lifts. The use of vibratory compactors and with hauling equipment, 95% Standard Proctor is readily achieveable these days. Getting closer to the top of the fill below the subballast, one my want to use non-woven fabric and better material for the top say 0.5m.
The above are not cast in stone and much depends on the nature of the fill. The term silt can also be a soil which has gravel sizes but has more silt sizes. It is often difficult without a gradation or look at the material, hence you are the judge. Look at the Proctor curve for moisture sensitivity of the material.