Thralls spawning and standing and simply fading from existence provides a lackluster experience.
I recommend thralls after the wild surge ends be given one of two characteristics:
Set up camp. Make the animation, rather than simply standing there, look like they have set up a camp.
Migrate to a better location.
a. A random location is chosen on the map at one of three distances; near, medium or far.
b. Thralls migrate as a pack from origination to destination fighting any creatures/players/installations they come across.
i. the destinations could be somewhat fixed (sentient beings look for fresh water, higher ground, shelter, etc adding “some” predictability to their destinations."
c. set up camp then fade after x amount of time.
This would provide justification/opportunity for base defenses, pets, thralling opportunities without necessarily forcing extensive travel; provide greater opportunity to engage thralls and and add some dynamism into the world.