The root word of the expression steeped in is the verb steep, which means
to cause to stay in a liquid, especially in order to become soft or clean, or to improve flavour
It generally has the connotation that you leave something in the liquid for a long time, and some of the qualities of the liquid transfer to the item in the liquid. When you apply that to the expression steeped in, you mean that a person or place has been exposed to the specified quality for a long time, and they have acquired some of that quality themselves.
The quality of uniqueness doesn't really work, because the town is itself unique, rather than having been exposed to uniqueness. It's questionable whether you could ever use uniqueness with steeped in, because how can something be unique if it gained this quality by being exposed to something else that was unique in a particular way. That would make two things that are supposedly unique in the same way, which is an oxymoron. You could say that a town has been steeped in history, meaning that it has acquired an 'aged' quality over time.
Similarly, the artists and writers are creative themselves, rather than having been exposed to it. You could say that a poet brought up by artists was steeped in creativity as a child, meaning that they were exposed to it before they themselves became a poet... maybe as an explanation of where their creativity came from.