So apparently there's a way to set victory conditions, AI difficulty, map size etc. It's a half-dimmed settings button right next to the very large and bright "START GAME" button so I will forgive myself for not noticing it sooner.
Restarting on easy then, just while I learn my way around, the AI is definitely less aggressive. What is not less aggressive though, is the constant raids on my bases by the locals. This happens all the time - it's a common complaint I've read elsewhere - so unless you're prepared to spend staggering amounts of gold and manpower fully equipping every conquered village with militia units, you're constantly playing whack-a-mole with your armies.
This well put most of you off even more, but I'm finding Dune: Spice Wars rather stressful to play. Besides the raider whack-a-mole you have to be on constant lookout for worms. If a worm appears near a harvester you have (I'd estimate) no more than ten seconds to realise - not easy when you're busy - and recall your harvester[1]. Similarly, worms will appear in the deeper desert too, always suspiciously right underneath my armies. Thus you lose armies for no good reason on occasion. It's not a particularly common occurrence but very bloody annoying when it does happen, especially since in early game at least it's very expensive to hire and maintain armies and they're slow to get where they're going too.
To add to the stress, you never have enough of things. With four distinct research paths, eight different ways you can split your spies, etc it's all too easy to be jack of all trades and master of none. You never reach a point - at least I haven't yet - where you can say your economy is ticking along nicely. If you're in positive rates for all your resources, you'll undoubtedly want to hire more armies which will deplete manpower, but you don't have the plascrete to build another recruitment office, and so on. It's never ending.
To a certain kind of player this is probably bliss. It probably would have been for me 20 years ago too. All I can say is there's never a point where I can just sit back and watch my empire. You have to just expand expand expand expand expand. If you want more plascrete for example, what are you going to do - research some tech that gives +1 production in each village, or conquer the next region which will give you a flat +40? And if you don't, your opponent will.
It's a conquest game really, and I'm willing to bet that even if you choose some other victory condition it won't make a lot of difference. Expansion is by far the best way to grow your empire.
[1] Actually you can set your harvester to auto-recall, but that decreases its productivity for some reason. This is silly. You can attach an ornithopter to a harvester but all that does is increase productivity - why doesn't it act like a spotter instead? You know, just like in the lore. Especially considering you can also add crew to harvesters which - yep, you guessed it! - increases productivity! Seems to me that if you're willing to spare a precious 'thopter slot on a harvester you should get the auto-recall for free. Gamifying it such that you have to keep a constant eye on it in readiness to hit "recall" with seconds' notice is lame.