The router would get you close,but most likely break the fade before you get thin enough.
Disc sander is good for the back of my bows(I grind that side before cutting the belly side),but won't do the belly side ramp curve(without seein your riser,just a guess)
Spindle sander is what I use for the belly side curve. I bought the cheap one from Grizzly,it is worth every cent,very very close to square(I check my fades both sides and both ends with a dial indicator at 1" and 2" from end)Well spent $140.00
Marty does (or did) his really well with a handheld belt sander!