Sigil--interesting that you would mention that city. In the City of Brass, there was an endless gushing of propaganda on how Sigil was a prison, a trap, a cesspit, “the cage”, a myth. And had very high prices.
If the “true” stories are true, Sigil is a central place in the cosmos that allows access to anywhere, and finding out about (almost) anywhere. But you can’t just fly there on an astral ship. You have to find a door (a portal) and then get through it.
In terms of controlling this particular cosmic opening. You are not sure. You know you can try to stabilize it so you all go to the same place. You are not sure you can pick that place even if you had some way of identifying it. And you don’t know if you could do that either (say, with a sigil sequence).