DT is also better against Tyranitar, since he resists Dark.
IMO, you should use the dusts when you get a DT/O or DT/HP since these are the most optimal choices and karps are common as fuck during this event.
Try to evolve a couple more until the event ends and see what you get. Worst case scenario, you got yourself some high CP gym slots hunter.
Save the B/HP in case Steelix gets a boost after the gym rework.