At the limits of what seems likely to be feasible with atomically precise manufacturing and chemical energy scale technology, you arrive at something similar to a somewhat slower Star Trek replicator, able to produce on demand in under an hour more or less any food item(s) and tools/knick-knacks/products anyone anywhere has ever created (likely food would mostly be its own machine). It's a world where disease is functionally eliminated, and it's plausible that barring accidents you might live for thousands of years. Energy is massively available from ultra-efficient solar panels and fusion reactors easily assembled by limited machines from parts printed out of industrial replicators. Used materials get recycled down to smaller components with very high efficiency, and damaged components get recycled down to molecular components and reformed into new ones.
What human agency, in the way that you mean it, exists in such a world that doesn't in the Culture? You don't have Minds, but you don't really need them for a world without disease, poverty, or scarcity to basically anything. No one's life would really mean anything, because everyone would have everything they could want. Eventually, we will have mastered all technologies that can be developed. The world will be finished. The only thing left is to wander around the universe in giant ships with all the comforts of home, playing games, etc.
You can get ~99.9% of the world of the Culture without superintelligence. Just fast forward current normal human development a few hundred years and you get the same world where all instrumental value of things ceases to exist, where very little remains to be discovered, and nothing you do is going to meaningfully change anything except what happens to you and those who care about you.
Of course even now, arguably, that's the case for virtually everyone.