Mmmmhh, I dunno; A couple of million drones scouring and cataloguing human-hospitable worlds for surface activity... Having the terraforming criterion narrows the search considerably. I'd assume their travelling is at least on par with ours, if nothing else from spying/captured craft but then, who knows - if, despite everything, they can't do instant hyperspace jumps, then... :7
This actually raises an interesting extension, which I guess may be where you were going with this, other than just the atrocity thing, and harvesting. Would humans "inoculated" (hey, early vaccines derived from mold - perfect

) with dormant mycoid fungi (maybe just not adapted to grow in us, maybe indeed stuffed into an engineered virus encapsulation) be flagged worthless as hosts to Thargoid larvae, or maybe even deliberately sacrificed for use as a sneaky infection vector...
...hmm, actually the frigid ammonia biology thing, and warm-blooded hosts of our sort, as used in the Dark Wheel, would appear two incompatible pieces of tentative lore, wouldn't they..? (EDIT: unless the change between stages is quite radical

)