As Surimi said, authoritarian slave armies are probably one of the best and cheapest ways to smother unrest - plus, during actual wars, it's hilariously easy to swarm planets with slave armies without needing to wait for a prolonged bombardment to soften up the defenders.
One thing (probably not working-as-designed) is that mid-high levels of Unrest often result in planet modifiers that increase the attraction of certain ethics... particularly authoritarian and militarist ethics, conveniently. Aside from an easily-repaired insurgent bombing or the odd riot here and there, unrest generally makes itself go away quite handily. As for consumer goods, I'd say just setting them up to 'decent' and grant them chattel-citizenship status (not residence) tends to cheer them up somewhat.
I can't speak to the alternate forms of slavery from the Utopia DLC, but I've found that default chattel slavery is surprisingly good at pacifying a newly-conquered population. Just make sure to divide the conquered POPs evenly between slaves and non-slaves. The 'unsuppressed' POPs might complain a bit at first, but they'll quickly gain authoritarian sympathies after a short while - especially since being on the same planet as enslaved pops will increase their attraction to authoritarianism. There's a particular humanitarian atrocity that happened in the 1990's that was attributed to very similar circumstances - but referencing that in detail would be in pretty poor taste, so instead I'll draw reference to Jane Elliot's
blue eyes/brown eyes experiment.
Essentially, the best way to divide an angry mob is to arbitrarily isolate one half, and convince them that they're somehow entitled to better economic/social treatment than the others from their 'mob'. This creates a social rift, and then unrest starts dropping since there's no unified 'voice of resistance'.
I cannot stress how important it is to achieve this balance: An authoritarian conqueror cannot handle the unrest of an entire planet full of free POPs for obvious reasons. However, unconditionally enslaving everyone on the entire planet will not suppress the unrest enough - furthermore, since all enslaved POPs are attracted to egalitarianism, planet-wide enslavement to fix unrest just 'kicks the can down the road.'
Thus, I think the best solution is dividing conquered POPs between enslaved and unenslaved, which slowly cultivates a local support base to help smother unrest from the other enslaved POPs. After all, "It's better to be the right hand of the devil than in his path."
Paradox games really bring out the best in people, don't they?
On the overall topic of empire expansion, I do find that it's easier to expand as an authoritarian when I turn defeated empires into vassals instead of trying to annex their entire territory outright. I just cherry-pick somewhere around 2 to 5 of the loser's most useful planets and let them keep the rest of their territory in vassalage. This means I don't need to sit around drumming my fingers for a full decade knowing that I'm going to need to gobble up the rest of their planets in a followup war, plus this helps me get the best parts of a defeated empire's territory without shouldering the extra science/unity strain of supporting small, useless colonies that the AI may have started. There isn't get as much of a direct mineral or military production boost from vassalage compared to outright annexation, but I've found that it tends to balance out when the vassals provide military support in subsequent wars.
Having a bunch of vassals is like being in a federation, except the overlord gets to be federation president 100% of the time, gets all the votes for war, doesn't need the approval of the other federation members to add new members (the overlord doesn't even need the approval of the empire they're planning to add to their 'federation', depending on how you look at it), and if a 'federation' member doesn't want to cooperate with the overlord, the overlord can just eat their entire empire using subject integration.
All this being said, I have found that playing an egalitarian empire with civics/ethics dedicated to boosting happiness is more productive and requires less work from the player - a POP's productiveness can be boosted as much as 20% without planet-specific micromanagement, across mines, energy, and research. Enslavement, if I recall, gives +10% productivity as a flat boost, +10% IF a planet's tile is used on the pricey slave processing facility (they take a long time to build and the Sector AI just loves demolishing them, so watch out), +10% if one of your empire's precious civic slots are used on slave guilds, and +10% if a leader slot is used on a governor with iron-fisted, and another +10% if the 'Share the Burden' edict is active. That is a hell of a lot, but it's only for mineral and food production, plus it takes quite a bit of time and dedicated infrastructure to make slavery profitable, and of course it doesn't apply to energy or research like a happiness boost does.
One mistake I think a lot of players seem to make is assuming that the core-authoritarian playstyle meshes well with a core-xenophobic playstyle, simply because they both permit slavery: the problem is that the core xenophobic approach tends to rely on slavery as a 'temporary,' short-term fix to keep conquered xeno POPs under control until the xenophobic empire is ready to replace the slaves with happier and more useful POPs from the empire's core species. By contrast, authoritarian empires rely on slavery as a perpetual, foundational part of their society, and must make sure newly-conquered alien POPs implement, accept, and eventually support class-based enslavement in a stable and productive fashion.
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