The Necrons.
Unfortunately, I know less about the various dynasties then I do the general Eldar factions.
One of the additional reasons (besides the ones I put in my post) that I put reconciling/allying/opening diplomatic relations with the Eldar above doing so with the Necrons is that the Imperium itself knows much less about the various dynasties than about the various Eldar factions. Your average Imperial might lump all Eldar under the same banner, but the Imperium as a whole has ~10k years of experience with the Eldar and people who actually study them do know about the different Craftworlds and their differing attitudes and philosophies, not just basic stuff like "Asuryani are not Drukhari and vice versa." But the Necrons only started waking up much more recently and have generally not been terribly communicative. So there's a stronger base of knowledge re: what these guys are about with the Eldar.
Another reason I put the Eldar first is that Pandora can out-Warp the Warp skills of the Eldar and hence can use her Divinatory abilities to effectively guide diplomatic efforts there. But the Necrons' schtick is just outright blocking all Warp effects, which means that out of every faction they're probably the hardest for Panda to bring her unique skillset to bear to help with whatever we're trying to accomplish with them. We should definitely try to get SOMETHING more productive than a mutual commitment to fight each other to the death going there, but as a probably more difficult challenge I'd prefer to leave it for later. Pick the low-hanging fruit first, yeah?
we don't actually know what an "STC" actually is.
FYI, I'm taking your post as a prompt to discuss STCs and the Ad Mech in general - I'm sure at least some of what I'm about to say is stuff you know and/or is actually addressed in some form in the vid you linked.
The explanation of the Ad Mech's technological knowledge base that made the most sense to me at un-grimderping the setting while still being reasonably consistent with canon went something like this (I'm probably going to fumble at least part of it, but this should cover the broad strokes at least). First, on the institutional level the Ad Mech actually DOES understand a lot about technology. That's why they can make crazy original shit like Ordinatuses when they feel it's called for, and why they're able to make so many different "patterns" of many items - let's look at lasguns as our example. There are many different kinds of lasguns, which have been altered and modified in many different ways to have different properties and abilities as weapons.
However, they're all built around basically the same lasgun battery. That's because the lasgun battery, while superficially the most humble part of the device, is actually fucking insane. It's a battery small enough to hold in one hand, but powerful enough to power dozens of laser bursts each strong enough to kill a man, that can be recharged quickly from pretty much any power source with very minimal drop-off in capacity or efficiency from repeated recharges, that can moreover recharge itself slowly from just being left in the sun, or recharge itself more quickly (albeit with appreciable damage to longer-term lifespan) by literally being thrown in a fucking campfire. That's CRAZY. And that part of the technology is basically a black box. The Ad Mech does not understand it, for the actually fairly good reason that all that should probably not be possible under physics as we understand it. And
that technology is what's drawn from an STC.
So, how come STCs have this tech, but the Ad Mech can't replicate it independently if they supposedly have good institutional technological knowledge? Well, the way this explanation goes, it's because STCs were originally created using AI computing that the Ad Mech is religiously opposed to using ever since the Cybernetic Revolt. In the Ad Mech's lingo, "AI" actually stands for Abominable Intelligence - any form of intelligence independent of a human mind is anathema to them. It's why servitors are such a (gross, fucked-up) thing throughout the Imperium, and why "cogitators" typically have human brains plugged into them rather than microchips. The reason why the Ad Mech can't replicate STC-derived tech is because the Ad Mech is committed to never using the tool that allowed them to be created in the first place, as a bedrock religious tenet.
So part of the dysfunction in the Ad Mech is that they're committed to chasing a dwindling stockpile of tech they will never be able to recreate themselves because they've (for at least somewhat good reason, to be fair) disallowed themselves to ever use the tools necessary to create them in the first place. And they're committed to that at the expense of devoting their efforts to building on the knowledge and science they are able to understand and use themselves. They've religiously elevated ancient technology so much that they're actively suspicious of innovation and new developments - in conventional Ad Mech theology, STC-derived tech is the true, pure form of technology. And the reason they can't replicate it isn't because they've forbidden themselves from using the tools necessary, it's because they're a divine mystery,
meant to be revered rather than replicated. So any kind of change must at minimum be extremely carefully reviewed for technotheological validity before being approved, because any change inherently risks further deviation from the singular perfect form of technology. Of course, like every Imperial faction, there is considerable variation from the conventional theology in many elements of the Ad Mech, but even then they are still hampered by the fundamental barriers to progress in the Ad Mech. Such as the superstitious theological cruft coating all actual scientific knowledge in the Ad Mech - even when they're conducting independent study and experiments, they're still filtering all their efforts through a perceived need to perform extended rituals of appeasement and the like.
The other huge element of dysfunction in the Ad Mech is that while on an institutional level they have a very high level of technological knowledge, they are very very bad at broadly disseminating that knowledge even among their own membership. The transmission of knowledge in the Ad Mech functions on something halfway between a master-apprentice system and a mystery cult, which means the highest levels of true understanding are possessed only by the highest-ranking members, and they are very reluctant to share. So much like the dwarves in WHF, this makes them extremely vulnerable to losing knowledge forever if these masters are killed or die without first passing their knowledge on. That's why as bad as the Horus Heresy was for the rest of the Imperium, it was even worse for the Ad Mech - they lost a huge amount of institutional knowledge at that point, because so many of their masters/high priests/archmagoses died without first passing on their knowledge.
She does, actually! Pandora started off believing she was just a minor Goddess of Magical Girls, but later either inherited or discovered that she actually had unrestricted claims to the entire Domain of Sacrifice, which let her tap on just about anyone. There are now plenty of aliens in her afterlife, including a great many who were wiped out as a result of the Imperium's pogroms, and just as many of those volunteered to become Deva as her human followers did.
That's pretty dope tbh. Good for Panda.
It's not a great policy to underestimate the Tau for being a relatively small polity, is my opinion. For a 50 Sector polity, they've held off a tendril of a Tyranid Hive Fleet pretty well.
I'm not sure where "50 Sector" is coming from, unless there's a source I'm missing; at least per the wiki, this is the current size of the Tau Empire:
The T'au Empire is composed of over twenty fully-developed Septs and around one hundred settled worlds, but the exact number and most of their names are unknown to the Imperium.
They are indeed doing very well for a polity of one hundred worlds. But compared to the Imperium, containing (pre-Great Rift anyway) roughly one million worlds, they're barely a blip on the radar.
For what they are, they're doing very well. But what they are is minuscule on the scale we're dealing with.
especially ones that are 'blatantly getting rid of political opposition' is only going to make us enough enemies we can't do anything to help anyone.
I mean. If any given High Lord
isn't blatantly getting rid of their political opposition, it's for lack of opportunity or means, not lack of willingness. Doing so really isn't a breach of established institutional norms or anything. If anything, it
is the established institutional norm.