So yeah, you can find some empty solar system and set up a base there. That's literally the first part of TIM Island, the easy part. The hard part is everything that comes after. Even if you don't buy steel but mine your own iron ore, refine it and put it through the chemical process needed to harden it up to spec (and do this for every component on the ship) you still end up bumping up against the issue of where do you get all the manpower and equipment from? That's thousands of highly specialized people gone just for the steel. Then there's tens of thousands more for similar roles and tens of thousands more for all the support roles. Somehow no one on the council, systems alliance or anywhere else starts wondering why so many people in potentially sensitive roles for the armaments industry are going dark? No one is wondering why a lot of highly classified, supremely restricted components only used in military warships are going missing (and no, you can't just make a factory to push out advanced stealth composites, military grade computer hardware etc. it is so specialized fields that it requires decades of investment before you can make something actually useful even if you steal your enemy's plans. If you don't you are looking at at least another decade of experimentation and prototyping, for examples look at the USSR and nukes or China and Iran and stealth tech) or why thousands of medium and large sized freighters are suddenly making weird detours into empty solar systems or providing fake flight plans? No one is apparently bothering with monitoring mass relays despite the fact that monitoring chokepoints is the very basics of military and logistical intelligence. The more you think about Cerberus ability to wage open war and supply itself with advanced equipment the more stupid it gets because it assumes, as Young points out, that the entire Galaxy is a bunch of passive dunces waiting for the plot to happen to them and not a place full of incredibly competent military and intelligence personnel who will be proactively go out to find threats like a terrorist organization with a fucking space fleet.
The entire problem with Cerberus as presented in ME2/3 is only partially that it went from "rogue black ops unit" like it was in ME1 into this massive, hugely influential organization with vast resources that no one can pin down for some reason, but has bases everywhere and somehow is on the cutting edge of pretty much every research. It is the organizational equivalent of a Mary Sue both in terms of powerset and how the entire story warps around it to accommodate it and give it a disproportionate amount of screen time. You needn't look any further then to the fact that you have more encounters with Cerberus then with the MFing Reapers in ME3 to see how Cerberus is like a cancer of bad decisions and storytelling that actively makes ME3 worse at every turn. I've said many times that Kai Leng could have been a really cool villain had a better writer used him more sparsely and less bluntly. The same is very much true for Cerberus. A good writer would have used them as a shadow enemy with sleepers that sabotage your attempts at unifying the galaxy against the Reapers and small squads running interference on you. Instead we get a terrorist organization with a massive fleet, a sizable elite ground force and an ability to attack any target at any time no matter how secret, strategically important or well guarded it might be. It is all stupid. It would be less stupid if ME was drama first like Star Trek, but ME was distinctly built on a details first approach to world building and in that context Cerberus is beyond the pale stupid.
. Of course all of that will involve formalising their army like everything else, but they've probably still got until the end of Liara's lifetime before they'll be allowed to progress in that area beyond what we've allowed Japan to do since World War II.
