Is there a particular reason to cap advancement at the level of d12?
In contrast, in Fate Core you can extend your pyramid to new heights for an additional cost. Translating to Demon Hunters terminology, we could allow moving the first die from d12 to d14 (or 14->16, 16->18, ...) if the Significant Milestone is at the same time a Major Milestone.
Granted, d14-d18 are not part of "the" polyhedral dice set but you can get them.
Do I break the game if I allow characters to move higher than d12, eventually?