When I have routed ash and hickory stringers,I start with a spiral up cut bit to remove the bulk of the meat. Then, with a second router, preferbally, to prevent bit changes, I repeat the cut with the dovetail bit. I have had relatively good success with this method. I learned quickly that the words Hickory, ash, and kumaru equal much higher priced stairs due to the blade/bit costs and the pure, unadulterated frustration of routing steel. Good luck!. Oh, by the way, did you make your own plow template, or are you using a store bought one?