Good attempt, but I would still jump into that sugar cage in a heartbeat (of one of my multiple redundant blood pumping organs).
So you still have freedom of self...just not much power to shape your civilization. Most of us don't have that anyway and at some level I think I'd be ok giving that up to a group of super intelligences so I can spend more time doing the things I enjoy.
In fact they do. Humans can call a vote on anything in The Culture - I would have to re-read some books to provide sources, but it was mentioned that sometimes it was about very trivial things, and sometimes more important. Mostly though, the humans were content to let the Minds do all the maintenance.
And yes, I do think I recall reading that some Minds are in fact made of previously-humanoid beings. Sometimes entire crews of them, if I recall correctly!
I think letting humans vote on some small things may be the equivalent of letting your dog choose which direction to go. It's a decision, but ultimately useless in the grand scheme of things. The minds may be an even greater cognitive leap from us than the difference between us and dogs. They can easily give the illusion of choice without relinquishing any real control. This is all speculation of course.
But on a smaller scale, the (still billions of) beings on, say, an Orbital can have a much greater effect on things. But again, they don't often choose to, opting to let the Minds run things. And, given that the Minds are sentient beings and citizens of The Culture as well as the humans, this is more or less the same as our current delegation of decision-making to politicians. Of course there's much to be said about doing that presently, but I think it's a given that at least the Minds are not incompetent, which can't always be said about our current politicians :P
And do keep in mind that it's not "letting" humans vote on "small things" - they have the right to vote on any matter they want, big or small, and as far as my reading goes, the Minds would respect that.