Heavily swayed by a recent reading of Nick Lane's excellent book on Mitochondria[2].
The fact that even our nearest neighbour probably has an earth-like planet is, from this angle, a bit depressing.
* [1998] http://www.geoffreylandis.com/percolation.htp
* [2014] https://arxiv.org/abs/1404.0204
One potential "great filter" always stood out as likely to me, even if all the other leaps before we came along had been made:
That filter is developing the combination of (i) self-aware intelligence to a degree that you are capable of deep layers of abstraction, (ii) the physical ability to adeptly physically build complex creations, (iii) an inclination toward social living that encourages cooperation. Other relatively high intelligences have evolved here on earth: octopuses, dolphins, elephants, etc. But it's hard to imagine any of them being physically capable of dramatically modifying their environment and building complex, useful creations. And we mammalian humans, dolphins and elephants are all social and cooperative to some degree, but octopuses don't seem to be. The combination of these three occurrences is extremely powerful, but may also be extremely rare. And it's difficult to imagine a space-faring civilization that wouldn't need all 3.
And Group 2 seems reasonable too. In particular, it's interesting to consider that while we currently hold an expansionist mindset because expansion has driven the course of our civilization, this may be a temporary thing. It makes sense when competing locally with enemies (other tribes, cities, nations, empires, etc) that are very similar to yourself. And we believe that survival and expansion go hand in hand. But it may well be the case that intelligent species generally discover that survival is maximized when not expanding aggressively, beyond some point, and when conserving resources. Hiding themselves so as to minimize the risk of contact with other alien species may go hand in hand with that as well.
So, between seemingly plausible filters in both Group 1 and Group 2 explanations, I think it's reasonable to entertain the possibility that it'll be a long time before we meet another alien civilization, if we ever do.
In other words, colonies lacking resources inside the expansion wave would start attacking their own neighbours, and colonies at the periphery may decide to expand inward, because even though other colonies' resources are harder to acquire, they are already processed/terraformed and therefore more useful. So you don't expand, not necessarily to hide, but to avoid creating enemies in close proximity to yourself, because you can't control your own offspring except at a tremendous cost.
Of course, even so, it seems strange that no civilization would ever make this mistake. Perhaps those that do are far away, or the propensity of colonies to eventually expand inward slows down the expansion.
This could be possible if interstellar travel was very, very, very easy. But in all likelihood the last place you're going to want to find resources is on a tiny speck of rock 20 lightyears away.
Furthermore, synchronization overhead scales with distance: you may be part of a hive mind, but if it takes ten years to exchange a message with the rest of the hive, you have to act independently during these ten years. This means the hive's "goals" must be replicated on each colony well enough to make sure they can't drift too far in that time. This is not cheap: for example, a lot of innovative technologies would have to be stifled while the whole hive assesses the risks they pose to its own integrity. Independent colonies, on the other hand, would just plow forward.
I imagine a hive mind would first send scouts to potential habitats, wait to gain enough information about a world from them, and then dispatch a colonizing group with fixed orders and some leeway in local operating parameters.
After that they'll coordinate just like all species will have to, within the confines of Physics.
If a colony cannot adapt to a planet while remaining within the hive's goals, then I guess it simply withers happily, till new scouts deem the world worth another try.
That could actually turn out to be an efficient way of spreading across the galaxy..
To put it in another way, I think an expanding hive mind needs to develop an immune system of sorts if it is to stand up against nimbler (virus-like) societies, but it's hard to see how it could succeed effectively without extended exposure to the latter.
So I feel there's a kind of catch-22 going on: if we expand without extensive synchronization, we create competitors out of our own offspring, but competition breeds efficiency, so even though the system is a horrible Malthusian hellhole on the inside, it is pretty strong as a whole. But if we try to avoid this and expand as a hive mind, we have to expend a lot of overhead on synchronization and an immune system, but we don't really know what the potential threats are, so the result will be brittle.
Interestingly, a planet on Proxima Centauri is a major plot point in the novels.
Nation states and / or races are a hopeful and cheery counter example... or in for a hard time.