I copied and edited the rules from my old old Age of Restoration II game. I'll mostly re-use them. The most notable change is that the early Trade research will give a slow income of Trade Goods, which you can't use yourself, but others can use as luxury goods. Trade should be more fun if it's not always zero-sum.
Rules PostCivilization and city name: The Reclaimers, of the Rusted Caravan
Race and society: The Bronzemade are mechanical beings, leftovers of the golden age awoken by chance. With shattered memories of the Calamity, they struggle with the simultaneous desire to reclaim the lost knowledge of that age and the fear of repeating its mistakes. To that end, they have collectively sworn themselves to pacifism as they travel the world in search of the ruins of the past. In terms of governance, hypothetically, each Bronzemade has a vote in their council; in practice, most will give their vote to one of the Caravan's philosophers and debaters who seems to most closely align with their opinions.
Theme: Wandering Pacifist Scholar-Merchant Archeologists
Brief History of the city and its surroundings: The Rusted Caravan is barely a city. For one, it moves. But it roughly fulfills some of the functions that a city would need in the slow-moving vehicles and heavy sleds carried along it, with resting quarters, repair workshops, and meeting chambers carried along in the procession, though the rough status of these vehicles render the Caravan unable to move into difficult or crowded terrains, and the lack of walls renders it difficult to defend the way a static fortification could be. The Caravan started in what could only be called a graveyard - the site of an ancient battle at the tail end of the golden age, where many of the Bronzemade fought and were killed or otherwise broken; indeed, many of the Reclaimers bear these ancient wounds.
Magic type: Artifice
Racial Frequency: Exotic
I can't balance rules enough to make special nomad rules, but I can make your big civilization bonus be a mobile colony. You'll still have a regular city, so please describe the terrain around it.
There will be bandits. Please be more specific with how pacifistic you'll be.
I think it would start needing repairs, costing about 2 buildings worth (Around 6 labor and 3 wood/stone), and a unit of spears so I have an excuse for it to "suffer damage and chase off bandits" instead of destroying it the first time it gets attacked.
Players start with 10,000 people. A normal colony costs 5000 population, some materials, and research.
Starting with half population would be crippling. I think it might be balanced if it costs 2000 population, though that still is a hit to growth. It would be able to gather 2 resources from a square if it moves 0-1 spaces that turn, plus feed itself. It would serve to extend supply ranges. It would slowly explore adjacent squares for free. While it can be used to gather rare resources instead of an expensive colony, it would only be able to stay stationary for a few turns before being forced or strongly encouraged to move on.
Will that work for you? Are there any complaints about the balance for it?