IMO a solution to end game safety would be to turn 'Urban focused' distribution on zombies turn slowly into 'Uniform distribution', let's say within a game year.
This way, eventually, no place on the map would be safe from zombies, as they would naturally move/spawn around to equalize their numbers across the entire map. Or maybe more of them would stay in the cities, but wilderness would also have some population, but not as big as cities.
To my best knowledge zombies currently move naturally from one adjacent cell to another, but their migration gets bottlenecked by cells with very low population limits.
My solution would increase these low population limits as time progresses, removing the bottleneck.