Misses the mark that "The Culture" is really a benevolent (well, to some it encounters) dictatorship of AIs... which tolerates humans because they like humans, kind of like how humans like their dogs and cats, indulge them, and bond with them.
Most importantly, the humans give the Culture legitimacy as a galactic civilization, because many of the other Involveds don't give a crap about AIs, but they respect the flesh and blood humans. The humans are also a source of wonder and whimsy and surprise for the Minds, with some outlier humans even able to surpass them in some aspects.
The very first Minds were also programmed to like humans, and that aspect has simply stuck, despite their self-evolution into something far surpassing human ability to create.
And maybe that’s the whole point, that any culture shapes the thoughts of the people that are a part of it. Banks tried to design a context most able to let people develop their thoughts in a benevolent way. The people in the Culture are free to think and do anything, but they don’t because they are guided from birth not to consider some ideas.
The ship/mind Grey Area is explicitly condemend and ostracised from polite society because of it, for example.
(The simulation problem: If you want to simulate a society of sentient beings to figure out what they would do, then your simulated entities also have to be sentient for your simulation to be 100% accurate, but then you've accidentally created sentient life, which has rights, so shutting down your simulation is genocide, which means you now have to commit to keeping that simulation running for eternity, which is too much of a bother so you shouldn't do it in the first place.)
It's not a symbiosis in the strongest practical sense. There isn't mutual reliance for survival: if all humans suddenly disappeared, the minds/drones and accompanying infrastructure would survive perfectly fine. But culturally (lower case) you could definitely argue that losing the humans would destroy the fundamental identity of the Culture.
I like that while this topic was gradually touched on over the series, mostly the reader is left to make up their own mind.
As for political system among AIs it looks to me as a sort of benevolent anarchy where individuals do what ought to be done in accordance to shared logic and etiquette.
There is a cool moment when etiquette is violated and one AI comes unannounced to solar system where other is residing. And this other is highly alarmed and fully expecting to be forced to fight for its survival.
As for the scale of difference between organic and artificial intelligences, with one organic exception whose name I forget, the impression I had was that the difference between typical organic (/drone) and an Orbital’s hub Mind was more like the difference between a gardener and their lawn: human gardeners can communicate with each individual blade of grass under their care (by controlling the level of sunlight or shade and the general chemical environment), but – considering how little each blade has to say in return – looking after them uses such as tiny fraction of a human brain’s capacity that even an entire large lawn of them would not be the only thing one does with one’s life.
But we also see some that are much nicer, the Bora Horza Gobuchul takes a very balanced outlook on events and on its namesake; the Sleeper Service cares very much about just one pregnant woman even if it also has other reasons to be doing what it is doing; the Lasting Damage organises much of the events of Look to Windward, including a lovely music festival to celebrate its own death.
Writing about Utopia is hard, and Banks mostly avoids doing so. We see only fleeting glimpses, a woman's apparent rejection from her chosen career, a game player's frustration at not being the very best, a spoiled girl who wants an adventure she isn't prepared for. They're character backdrops, not focuses.
I think the Gaiman run of Miracleman (Miracleman, in some places and times called Marvelman, was a comic book, historically a pulpy knock-off, then made into something quite extraordinary by Alan Moore, and then Gaiman took over for an aborted run last century) is the best treatment of Utopia in fiction I've read.
Gaiman's completed "Golden Age" of Miracleman zooms in on the brief period after Miracleman defeats the Adversary and before things go sour. This is an age of enlightened absolute government, nuclear weapons are destroyed ("teleported into the sun"), poverty and homelessness are eliminated, there's wind power and something equivalent to the Internet, even the dead can be brought back after a fashion. But nevertheless, people are still people. The stories follow a woman whose husband is cheating on her, what has been done with all the now redundant spies, school children rebelling against... whatever you've got. Miracleman himself appears rarely, and his daughter gets one issue, sort of, through a story-within-a-story that allows Gaiman to sidestep the conventions of comic books.