Actually, for Ange, the Chaos Stone Farm no longer holds any value as the environment is poor and the area small, rendering it impossible to expand the cultivation scale, and without sufficient water and fertilizer, Ange can't optimize and cultivate them.
Moreover, it is located on the patrol route of the border defense forces. Although Nagi used some tricks to divert the patrol routes, it's better to be safe than sorry. There's always a risk of exposure, and if someone from above comes down to inspect, these tricks will be ineffective.
Ange has already planted the Chaos Tree in the Forest of Foehn Wind. With the protection of the World Tree, even if someone enters the forest, they probably won't be able to find the Chaos Tree. The forest mists can make them wander in circles.