Good point.

It would be cool to have a DMX channel allocation where the DMX level would determine the stacking order. (Obviously with a logic exception where when two or more mixes receive the same stacking order, the highest number takes precedence.)

As far as a preset solution, this is what I have used in the past:
preset n Mix 1 On, preset n+1 Mix 2 On, ..., preset m Mix 1 Off, preset m+1 Mix 2 Off. This way you can activate mixes in a specific stacking order by triggering different presets.