uhm this brings me to an other idea:
you could make a Panel (when you start the campaign) to adjust different Things in the campaign to adjust the Dificult Level.
So on the Panel could be:
the Enemy has better Techs more Energy / more Energy per oil derric Builds quicker.
Units Landet from a Plane have better / worser Tech Levels.
Enemy units have more hitpoints rof damage and so on.
Changing the AI if there is a Better.
This Changes could be also Applied for the Player. +
Early more /less Researchable Thinks (higher / lower Tech Level)
So you could give The Enemy units 4 times The Life Points they would have instead and you could give yourself one or two tech Level more to be able to handle it.