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2. The Culture books are critical of the utopia. More than half of them are directly about the difficulty of reconciling the ideals of that utopia while coexisting in a universe with other people. The subgenre the Culture books belong to is literally called "critical utopia" fiction.
3. All (good) sci fi is political. You should find a different genre if you don't want politics in it.
I think you are reaching one of the limitations of the English language here. Machiaveli's Prince and John Knox's Monstrous Regiment of Women are both "political" books, but in a very different sense. The former is an exercise in trying to understand the nature of power and society in specific circumstances (in particular, the Prince is a study of autocratic power by a committed republican). The latter is just a polemical weapon, designed to advance some political goal. When people complain about politics in literature, it's usually because they don't like reading the second sort of book. That sort of books are seldom good, whatever their genre may be.
(I'm intentionally using Renaissance examples here, to avoid any unproductive discussions on more modern books.)
Kameron Hurley has a longer piece on what it means for writing to be political:
https://locusmag.com/feature/kameron-hurley-the-status-quo-i...
And honestly, you can pick the "good" writer of your choice from Asimov to Zelazny. Their politics come through in their writing. Foundation and Lords of Light are both obviously political works. I don't need to get into Heinlein or Bradbury, or poul and it comes through the space between in the lines in Wolf and Pohl. Le Guin and Clarke wore their politics on their sleeves. Etc.
I'm not making some pedantic point here. Science Fiction is a deeply, inherently political genre and always has been.
If it was criticism of the utopic ideas, it would be just as much political propaganda, just for different ideas.
See also Use of Weapons and Excession.
(I wouldn't call The Culture "leftist" exactly, as it seems to be a space version of liberal imperialism, which the international Left always opposed. Though it is "left" in the American Fox/CNN-tier sense.)
If I choose to believe Banks' quote, then I have to interpret The Culture as merely the best possible society, in a universe where human nature severely constrains what societies are possible. To me it seems to be a world full of pointlessness and barely-suppressed existential angst -- so perhaps he is saying: "Imagine that we can solve death and supply unlimited entertainment and pleasure. Even then, you will not be able to escape a kind of background noise, an omnipresent drone, in a minor key -- because that's inevitable to existence."
Now I imagine a back-story: The people of The Culture, the Minds, everything -- they are actually in Hell. Their universe is Hell. They are incredibly clever; they have outwitted the gods (to the extent possible); they have ended the tortures, chained Satan (or whoever used to administer), and found ways to roast marshmallows over the lakes of fire. It is a tremendous triumph. And yet they are still in Hell, and no matter the palaces they build there or the drugs they take, there is pain beneath the surface.
I read The Culture books as a subtler version of that story, where it is never spelled out clearly where they are.
At the same time, don't take this to mean that I want to force some religious meaning onto the books. My meaning is also secular. Hell is a metaphor of some kind. The meaning I am suggesting is more existentialist than religious.
In most religious depictions, one of hell’s predominant characteristics is its inescapability—either logistically or by the mental limits of its inhabitants. Tons of Banks’ work, on the other hand, has a through line of “people could choose to leave the Culture, they’re fundamentally in their situation by (extremely unconstrained!) choice”. This comes up in the idea of Subliming, being Stored, many characters’ choices to commit suicide/decline immortality, or just plain leaving the Culture as many do.
Plenty of his characters are in hells of their own making/choosing, or are tortured by other people, which I think is definitely a commentary on the sources of suffering being behavioral rather than just environmental, but I’m not sure it follows that “therefore the Culture is hell”.
I'm not sure why you wouldn't just take what Banks says at face value. Living in the Culture would for the majority of people on on Earth be far preferable.
But I do think you're close to correct about what Banks was saying on the darker side of the message. Every book of his that I've read (including the non Culture novels) has a darker side. It always seems to be about the inevitably of conflict in the presence of the human condition. And the terrible cruelty this can result in.
Nevertheless, try as I might, it confounds me how anyone could look around at the state of life on Earth today and think "yeah, much better than the dystopia of The Culture".