You know what would sort out the difficulty level and the maths in all this?
Is simply to manufacture new dices. Change the symbols on them.
FFG is good in marketing they could simply call this something like
"expert dices"
And this would solve everything simply without us trying to sort out solutions
for the maths to work etc...
They could for example:
on blue dices take out one success or boon.
on green dices take out one success
on red dices take out the side where you have 2 boons and replace it by a success and an exertion
on black dices add a challenge
purple dices would stay as they are. (or simply removing on a face a 2 challenge so that would make sense to add more purple dices as challenge)
I did not check the maths here or anything. Its like a first sketchy idea.
voilĂ !