Oops, yeah, spoiler warning-ish? I'll try to be a bit vauge, but...
Wait a minute...if my logic circuits aren't failing me...doesn't this meant that if the spawn threshold is set low enough the Dark Spire will be in a perpetual snowball? 4 guardians die, prompting a Dark Spire spawn...the spawn itself is either killed, or destroys more ships, prompting a new spawn, and another, and another. All you really need to do is to trigger a spawn somewhere deep in AI space and watch the Dark Spire consume the galaxy? O_O
As far as I can tell, the spawn itself does not contribute to the threshold, so killing the spawn before it kills stuff (or killing it on a different planet) ends the chain.
Out of curiosity, was the devourer on? I expect the interactions of those two could be... devastating.
So I went there. Things were fine for the first 30 minutes or so. I built up defenses heavily, never left the homeworld for any reason. The devourer crossed several DS worlds during this time span, without problems. Things were killed, stuff spawned, then died. Finally, the tipping point was reached - by accident. From what I can tell, the Devourer went to a system adjacent to two DS systems. Each DS system had a bunch of AI Tackle Drone Launchers on it. The Launchers started producing drones, which attritioned out in a few seconds. This caused spawns, which killed stuff, which caused spawns, and so forth.
By the time there were 100,000 Dark Spire filling 3/4 the galaxy (the last cluster was behind a Black Hole Machine), the game was so slow that most ships had no orders and basically stopped attacking. By the time there were 175,000 Dark Spire, even the lowest performance profile and +10 was getting about 50% speed. This is also about the time ships finally began to enter my home. Lots of fighting, killing, etc took place there. Performance issues are causing more trouble than enemy ships by this point. And that's when the game crashed.
Basically, by the end, every AI reinforcement cycle was causing 1200 DS (40,000 strength or so) to spawn.
EDIT: Not the reinforcements, it seems, because the logs say "CANCELLED because global ship count >= 125000". But they were still getting 1200 ships every 2-3 minutes.