However, having fleets just magically appear the instant I drop a colony down
(I've seen this mentioned a few times and this absolutely *does not* happen in vanilla. It'll take at least 30 days of the colony being in existence before any "hostile activity fleets can spawn. It's possible that there's some kind of bug here, but I've not seen it, and I've recently spent quite a bit of time watching hostile fleets spawn in-system.)
They absolutely do exactly this in vanilla - I think what we're seeing here is a disagreement on what counts as 'instant', because of the way a new colony needs to spend time building its space port, combined with the fact that you can't install or uninstall AI cores and colony items unless you're physically there, which results in the player having to just loiter around a new colony until that initial construction is online.
(And then you have to loiter
some more because you've got no defenses yet, so some enterprising pirate base is
almost guaranteed to launch a raid within the first month or two, so there's really no point in going anywhere because you'll have to turn right around and come back, and hey look pirate fleets are appearing out of the ether now and giving you trade disruptions because hey, it's a new colony, it can't launch trader fleets that have any sort of ability to defend themselves. 'Instant' is relative, and may not be
literally accurate, but it's certainly how it feels right now.)
... those fleets should jump in or spawn from an existing in-system colony.
They generally do, or at least spawn both 1) outside the player's sensor range and 2) towards a jump-point, making this plausible.
This is going to depend a lot on the star system, IMO. Sometimes, yeah, there are a bunch of access points and it's plausible. Sometimes there are just two jump points, right near each other at their current orbital cycle, and the player's been right there and there's just
no way a pirate fleet could have gotten all the way over there. And it only takes one of those to make it obvious that the AI is cheating, and an obviously-cheating AI is
annoying even if most of the time it's not
that unreasonable.