So in theory from what I have seen, your story is that of a career pilot, with intro cutscenes and voice acting etc. This has all been shown already.
If you find it most fun to haul cargo, your story is that you will go get the licences, grind the smaller jobs. For more casual fun experience you might choose to specialise in agriculture jobs which are going to be more engaging/shorter.
The truth is though, it is a simulator, and flying takes fooreeever. You can't make casual flying and flight sim meet in the middle without throwing away 90% of the flying experience, which is level flight at altitude on your way to a destination.
This is why although I am an aviation fan, and a train fan, I don't play either sims. I have not got 2-8hrs to hurry up and wait.