Look, you are putting all the eggs in one basket. Any attack that disables or blocks the Gate traps the garrison. Furthermore, there is such a thing as planar leakage through Gates, which is the reason the Inevitables are tasked with shutting such things down. What that means is, ultimately, that the barracks will sooner or alter take on the same planar properties as the Plane of Fire. Maybe gradually, maybe spontaneously due to a weather event or similar.
What we need are multiple, independent and self-reliant barracks to ensure that neither sabotage nor targeted attack can trivially wipe out the whole garrison. And since a demiplane that is connected by gate has no guarantee that the planar effects of the PoF are kept out indefinitely and might in effect kill everyone in here in their sleep due to a simple mishap, we should just plan for people to stay on the same plane as they are stationed on.
As for material transportation, that's what a Terminus is for. We can build a station underground in the mine shafts and move both people and material in large quantities without needing PC attention, or permanent connections that could leak elemental fire and attract Inevitables.