I am still on my first campaign. When I found out that the story line was moving the difficulty of all other planets, I stopped doing the story mission.
It just happens that the lowest difficulty planet is 2 out of 5 skulls. And with half-skull intervals in difficulty, I was able to mod the test file (json) to make up to 10 contracts per star system that ranges from 0.5 to 3.5 skulls.
There are 4+ skull difficulties, but they seem to be reputation level locked.
Anyway, I've travelled to the western-most planet and back doing missions, and over 100 hours already. My heaviest mech are Orions.
I've enjoyed doing 0.5 to 1 skull missions using 3 medium + 1 light mech for a long time.
Then was running 1 heavy + 3 mediums (or 2 med + 1 light) around 1 to 1.5 skulls contracts.
So as I get new heavies, I've played each for a while learning how to use them. Dragon, then Blacknight, then Jagermech.
For 1.5 to 2.0 skulls, I use 2 mediums and 2 heavy as I became more comfortable how much punishment they can handle and dishout.
Also, during these times, my roster skill needs favor 2 types, so I recruit to fill alternate roles.
And then there is this cool +xp per day, so I hired recruits but do not assign them skill until they have around 34k+ exp to spend.
2.0 to 2.5 I use 3 heavies and 1 medium.
At 3.0 skulls, the 3 heavies and 1 medium are near death's door when multiple heavies with supporting LRMS and PPCS shoot out of vision range.
I have yet to get my first assault mech.
And the ++ weapons are rare enough that it's a delight to find them without tweaking the json for more to drop.
Now I am tempted to progress the mission to get into 1 Assault mech and do 3.0 skulls contract.
But I want at the same time I want to travel to the east end of the map.