later we will slow it down and use wood, hard leather or aluminum
Keep in mind the fact that the dusack is a wooden weapon, and thus using a wooden dusack "waster" is really akin to using a real dusack (though I'm sure they would have been weighted specially somehow). As far as I know, there is only one antique example of a metal dusack out there. Maybe you should incorporate into your sparring the idea that just because you got whacked with the dusack, it may not be disabilitating. So as much as you thing of strong strikes with, say, longsword, remember, wood doesn't (usually) cut, and wouldn't cut like a steel edge would.