The only planets I can't see staying independent are important (manufacturing) worlds like Hesperus II, Irian, Luthien, etc. They are simply too tempting not to invade.
Let's say Irian decides to become independent and stay away from all that fighting going on, so they can become even wealthier by selling weapons to everyone around them. This might work for a while. They use diplomacy and trade to keep all those forming regional power blocs from conquering them, pretty much trying to pull the same stunt ComStar did during the canon Succession Wars. But, sooner or later the regional powers will have been consolidated. Irian is still a heavily industrialized world, but they simply don't have the resourves at their disposal, that the bigger powers surrounding them have.
The reginol powers know, that conquering Irian will not be easy, but all those factories make for a target too good not to conquer. Sure, they could just keep buying irian's stuff like they just to, but that would mean letting their rivals buy military hardware from Irian, too. Also, no one can garantuee, that Irian will not at some point join one of their rivals. Best to act now and secure Irian for yourself, than wait until some one else comes to the same conclusion.
At this point, Irian has to options: 1) join one of the big powers on good terms or 2) try to stay independent at any cost. The Irian militia might be good and well equipped, but it has only the resources of a single planet at their disposal.
Let's say power A attacks Irian, but Irian is able to repell them and refuses to sell power A any more weapons. This harms A, but Irian's militia has been weakened. Power B thinks, that Irian is now weakened and will make for an easy target, so they invade. Irian is able to defeat them, too. Retreating, B decides to destroy as much of Irian's industry as possible, so they will be unable to sell to their enemies. Irian's militia is further weakened, plus their ability to rebuild their forces has been reduced (and they lost to important markets to sell their goods). This will repeat until someone conquers Irian or everything important has been destroyed and Irian is not worth fighting over.
The only way for a world like Irian to stay independent and keep their factories would be, imo, to become a respectable power them self.