Hey there,
I've done extensive analysis of all the missions and played all of them on Much Harder so hopefully you feel my input is valuable.
It's quite hard for me to find out what mission numbers are actually used for your maps. I'll name the ones that I think you mean...
In the third mission that you're talking about (Yorkshire), there are three teams. You + Mac; Artix and Ventrix; and the Splinter.
The Splinter has the highest difficulty AI of all of them, but is on his own.
Artix and Ventrix will attack both you/mac and Splinter.
The AI never does the same thing twice (Aside from obvious openings like making a factory and getting metal nodes etc.)
This means that it could well be that Artix and Ventrix have severely weakened the Splinter while you and Mac have become powerhouses. The opposite is also true, if you and Artix fight a lot, it could mean the Splinter kills Ventrix, because he is the strongest AI.
There are a lot of variables and these are all true even on Much Harder difficulty. I've seen a streamer play Yorkshire on Much Harder too, he got killed like 4 times in a row and the next time he tried it, ventrix KILLED the Splinter while becoming severely weakened because of it. This allowed the streamer and mac to just steamroll the mission.
So in the worst case scenario, the splinter and Artix/Ventrix do not attack eachother and only you/mac. This then becomes almost impossible to beat (If at all, considering you barely have any tech and orbital abilities).
For mission 5 (Decanus), you can decide not to drop the engineer. The AI does not take the points where you are supposed to drop the Engineer nor the point below it. Only when you drop the Engineer, will the AI suddenly become aggressive and try to kill your outpost.
This means a few things, the later you trigger the event, the easier the mission becomes. You can build bombers and AA way before that and just power up all the way.
For Mission 6 (Noctus) (And pretty much any other map from here on out) you have to use orbital nullifiers. The enemy AI will exploit their orbital abilities to the maximum extend that they can. Saving up dreadnoughts and leveling them up is very important to remain cost effective. "Creeping" forward with orbital nullifiers is the way to go once you start encountering Agethon. Before that, you can spend your resources elsewhere.
Now with the Refinery, you can use that into a late-game advantage. On the easier difficulties, the AI actually gets nerfed; from Normal to Intermediate to Easy respectively. The opposite is also true, the AI will be a powerhouse on Much Harder and you will have to use orbital nullifiers and a ton of anti air to keep your nullifiers safe. The second you lose one, you're likely to get an orbital bombardment.
I hope it helps, let me know if you'd like some more info.