With regards to FSG:TG, I'd like to offer a humble solution.
Let them go in any direction forever, but give your sensors a finite range (because any sensor has a maximum resolution, and going beyond that point would put things beyond your draw distance). Once you have reached that point, have them hit an invisible wall, but because there is nothing out there, they dont know it. Space is a very empty place, so just make the particle effects fade to nothing the farther you get from the objective.
The player would always be able to get back to the objectives, even when they were beyond his scan range, because the enemy ships all have IFF transponders, and their broadcast power is many orders of magnitude beyond your own.
Your objectives are within the sphere of the enemy ships scan range, so leaving that range serves no real purpose, beyond simply curiosity. Once players realize that there really is nothing interesting in open space, they will return.
As another overall game-flow idea; why not allow the players access to maybe half of the objectives, with the options to do them in any order you want.
I.E. - A single area has maybe 3 tier 1 objectives, 3 tier 2, and 3 tier 3. Each offers a different method of achieving it (Pure stealth, decoys, distraction followed by snatch&grab)
Some are dead ends, others offer multiple subsequent options upon completion, others offer only one path forward. So you would end up with 2 successful outcomes, all with different rewards, which would then help you on later missions. Maybe they would reflect the manner in which you completed the objectives? i.e. using decoys would reward you with better/more varied decoy methods.