I don't think the Thargoids have been a spacefaring race for 120-odd million years (the time it would take for something on the far side of the galaxy to rotate around near us)? If so, it's remarkable how un-advanced they are; and how they've suddenly gained advances in the last ten years.
We do know they were a space-faring species a couple million years ago, as opposed to humans for only a couple centuries. And we learned recently that Thargoids have the ability to travel vast galactic distances relatively quickly. Certainly the Thargoids don't evolve very much over the millenia. And aparently they only have one space probe model.
Maybe Thargoids go into hibernation... kinda like the bad-guys in Stargate Atlantis... and we just woke them up.
Or maybe the Thargoids are the mentally stunted idiot slaves of a super-powerful race that remains hidden.
Or maybe its a video game with limited time & budget for development.