Hey Dakiu.
Sorry I dont have the function plugin so I cant look directly. But did you know that if you right click on most events, you get the option to "invert" them (make it the opposite rule).
So if you had a marker where you were spawning enemies, you could invert "Enemy is Overlapping Marker" - which would of course mean whenever the enemy 'isnt' overlapping the marker -> spawn another.
Another idea is to have them spawning after a set periods of time. Maybe once every 10 seconds?