For me personally, this approach would restrict the toolset that DH gives me too much.
For example, say the Tier 2 party wants to recruit a criminal to break into the lord’s manor, and their prior reconnaissance has determined that certain specific valuables are held there, which they let slip to their contact.
I would set the difficulty at 10, then the Slyborne Wizard would get further advantage on the roll because he is negotiating with a criminal.
The alternative would be to set the difficulty at 14 for the tier, adding advantage because appealing to a criminal’s greed is effective, but the party would lose the benefit of the Wizard’s Slyborne feature.
Moreover, without the Wizard’s Slyborne feature, maybe to party decides the Ridgeborne Bard should make the roll, despite the fact that it is more narratively satisfying for the Slyborne Wizard to do so.