I already suggested it, but i think the difficulty is better balanced by changing the actual missions available to the player. I get watchtower mission in april of year 2, 4th difficulty, why? D level bounty hunts, rodeos. Remove them completely from the pool in like 6 months max. Basically once you hit the tech level to make the mission 99% safe then remove it from the pool. Like warrior armor+gun emporium removes ratmen rodeo. C badge + one of the tier 2 BH awards removes D level missions. And the same for ships, basically. Once you raid a few excavators maybe the factions should realise that you have a decent radar coverage and it's not not safe to use the rubbish vessels. Like if you have downed X excavators or did Y landed assaults then this ship is removed from the pool. Scouts should switch from cutters and runabouts to faster ships then if they keep being downed they should abandon designated scouts eventually and just scout with battleships.
This way the the overall game difficulty would be more like a campaign progression speed, instead of having more enemy mooks with super stats. I know it aldeady affects some aspects of campaign progression, but i would make it more radical.
You say it all like it was:
a) actually needed (the game was not created to entertain people, but to simulate a mysterious world).
b) easy to implement (what you propose is an insane level of manual-adjustment that would not only require huge workload, but also will be prone to endless bugs).
c) not implemented already (easy missions do get phased out, but slowly and mostly with clock, and almost never completely).
d) not already implemented with actually important missions which do depend heavily on diff level and tech progress.
I don't want people gaming the tech tree too much to avoid researching stuff just to keep some missions longer. That'd be madness and a huge blow to immersion.
Can't you just ignore these missions?
...Actually this is the most baffling part. Is someone pointing a gun to your head? Just ignore missions which you consider beneath your dignity.