I haven't started my 3rd go-around on this game, but I have made 2 serious attempts at the game and both times I came to the conclusion....the game would be much more open if you could advance levels without playing story missions....As it is the story missions do box you in and give you little latitude.
If you are really set on having the open ended side of it before you complete the storyline, then download the FLMM, (Freelancer Mod Mnager) found in the downloads section here at TLR. Then downloand one of the many versions of the open SP mod, and activate it using the FLMM.
Viola! you have vanilla Freelancer, with no story to get in the way of your explorations.
OR you could just connect to one of the many Vanilla, or mostly Vanilla multiplayer servers. You get openendedness there too, as Multiplayer has no storyline.
(Of course I suggest my server, Adui's Place, but Im predjudiced )
Just another day in Paradise!