Unfortunately poisoning this many mice in such a wide area will undoubtedly lead to wider ecosystem casualties to creatures who feed on live mice and carion.

This can be mitigated by using poisons that break down while doing their job and are only effective at specific dosage to bodyweight such that they naturally dillute while killing mice, but since the mice will eat each other that also means they will build resistance to it very quickly.

The most ideal solution would be a mass sterilization, such as catching and spaying almost all the females, castrating the males, causing them to outcompete themselves for food and shrink dramatically in population.

Barring that, a better solution to poison is to trick them into falling into a container, this works well with buckets and then they can be sorted and disposed of later.