Keeping on track with the theme of tracking…
Use another 4th tracker to track the completion events of the 3 objective trackers.
That is disappointing the prop manipulator doesn’t do the job… and funny it counts as a destroyed prop in the tracker …
I agree with @Astrotronic on using the Destroy Spawner function for the workaround
You should create a feedback & requests posts for adding an On Creature Spawner Destroyed event to the Creature Spawner, I’ll add a vote.