Yes, they will need to balance it where movement isn't free and fuel actually matters. And yes, you can build outposts - now you're understanding. Weak, lightly defended outposts that are rich with materials. You can expand, too, by making lots of outposts, each time making any individual outpost not as strong, because you didn't consolidate resources and defenses. As you expand in 3D space, the inverse-square law dictates how well you will perform. There is value in making many outposts, or consolidating all resources into a capital city, and even both. But all of this takes time, effort, coordination, efficiency and skill. It doesn't matter how many outposts, or how many people, but it does matter how much fuel is spent to move around the game world, and how much you get from gathering. So you are right, we don't know how fast fuel is going to deplete, but if they balance it where the fuel cost matters at all, then you hit that law eventually. In fact you hit it every time you go out, spend 1 minute flying to mine an asteroid, 2 minutes to fly back (you weigh more), and the next time you go out, it's 2 minutes to get to an asteroid, and 4 minutes back, and so on.
Likewise, if you have an outpost under attack, and send 1000 ships to defend it - and there are only 10 attackers who faked you into thinking it was a massive attack - you wasted a lot of fuel for nothing. Efficiency becomes much more important than leveraging brute force. It is not something you can combat. You will have to bend to the will of the inverse-square law, which will cull inefficient large groups, and only the most organized and efficient groups will maintain status as superpowers, just like in real life.