I don't think we need to revisit level sync at all. If anything, we should scale down equipment MORE.

The point of having level sync is to help newbs do content without them being carried. The dungeon is balanced around lvl 20 abilities, not only from a damage output standpoint, but utility as well.
As an example, let's take SHIELD OATH.

It doesn't matter how much you scale down shield oath's effectiveness, the dungeon was designed for the tank having ZERO extra defense from such abilities.
The same can be said about the utility effects of every skill in the game. Your character is MEANT to be less effective and less versatile overall at that level. Having that will invariably make you more powerful than an unsynced lvl 20 char.