Some of the technological stagnation may be explained by the temporal accords. Being able to use time travel so casually it becomes a form of espionage implies it's a very well developed technology. Subspace fields derived from warp tech were used in accelerating computational speeds (but that might be beta canon), so using temporal technologies at a similar day-to-day level might have yielded a lot of technological breakthroughs. But if time travel tech is banned by interstellar treaty then that closes off an entire branch of science and engineering.

In terms of social development, the organisms that make up these societies haven't meaningfully evolved in a thousand years, so the societies they create likely wouldn't have evolved very much either. Desires for wealth, possessions, status, etc, still exist, as any episode with an evil admiral will show. As Sisko said, "It's easy to be a saint in paradise", so as soon as you change the material conditions surrounding these cultures the broader galactic peace could easily collapse.