I remember i once went to mine some Sarthium and it was in Zirax territory, there was a bunch of Zirax soldiers around it, so i thought all right, i will just use my sniper to clear them out. No dice they would respawn just as soon as i killed them, i was just wasting ammo to be honest, so i decided to instead burrow beneath them with my drill. This makes any sort of tactical approach impossible, since you are always competing with the endless swarms of NPCs, until you destroy their base they just never stop coming. So here's my suggestion, make it so the NPCs have a cap on how much they can spawn in a certain area, this cap would slowly replenish over time as the faction brings in reinforcements to replace losses but in a short term it would be possible to clear out the area of enemies for a few hours, this would allow for more tactical approaches like sniping them from the distance rather then just having them respawn instantly.