Even considering the billions of humans spread between the various systems, how many ships do you expect them to have?
The actual creatures Thargoids, yes...I expect there to be potentially trillions of.
But if they have millions of warships, then I'd expect them to be spilling over en mass into systems -- and not in singles and pairs.
There may be resource limits even if the warships can be built quickly.
For any OXP makers that want to add more Thargoids to interstellar space...or systems for that matter...they have it easy, beyond the possible problem of the Thargoids looking like they just arrived when you do.
But to remove existing ships via OXP scripts, I don't know of a good way to do it without causing weird visual artifacts.