You should be treating 'turn undead' into a lamp that keeps the grue at bay.
This was Adam's insight that made the move click for me.
Damaging undead, or flat out killing them, takes a different sort of move I think. I tried to sell Adam on a fictional trigger of "When you touch your holy symbol to an undead creature..." but that's a different move, not Turn Undead.
The issue with making Turn Undead too powerful against undead is that it makes undead fights very binary: you turn them or you don't. By keeping it a bit more dynamic there's room for interesting action.
Also keep in mind that we're not using monster levels anymore. This has lead to some great changes in the rules to my mind. In particular, a monster has to be fictionally threatening in some way, not just higher level. Likewise if you want to say what undead can be effected you have to use only traits that are clearly present in the fiction, not just level, which makes something like "destroy any undead with 2 fewer HD than you" a non-option.