I have made a number of custom controllers for the H9- And sadly, I don’t think you will find that one size fits all here.
However given the routing options ON THE H9- you really just need some buttons and some expressions, and they can get assigned per taste.
I think the key, most common /elegant use case is this:
Bypass and Tempo – use the H9s own button for this. That way the tempo light works.
External buttons- I think 3 at minimum:
HOTSWITCH– an absolute must. Also must be programmed correctly for proper function.
Preset up / Preset down Obviously needed.
So for your controller- in addition to whatever you are up to with the exp pedals- minimum 3 switches for the above would be essential imo. Maybe a couple more if you have the space for favorite presets or the tuner? But three minimum.