I suggest borrowing the House rules from the Song of Ice and Fire. It's a solid system and it would easily allow you, as GM, to create NPC strongholds that the PCs could ally with or have to struggle again, Walking Dead-style.
It has rules for defenses, troops, and resources built in. Just change "wealth" to "supplies" and set a cost per time instance. There's even Population and Law stats, which you could make "Survivors" and "Order". The more survivors, the more of a bonus to improvement rolls but the more supplies you have to burn through. The better the law, the less conflict.