Duffy is a good call.
Other than him and Wacha (who will get Kluber money, maybe for a couple years) I don't see a SP who would be better than Kikuchi or White, in my opinion. There are a few guys like Cueto/Grienke/Davies/Miley/Bundy who would maybe be sideways moves - they would not improve the team as projected but would push everyone down and add depth I guess. But if you add a boring SP5 and push Kikuchi to the pen, who gets optioned? Or do you just bet on at least one injury before camp breaks?
I agree with you though that focusing on MiLB possibilities makes more sense. If you can get Lindblom, Pineda, or someone like that on a MiLB deal it does just as much as signing some bum for the SP5 slot and it helps with roster construction.