I knew about these people, you can find out about quite a few by playing historical games. There is one that shows them in it as a regional unit type, Europa Barbarorum I for Rome Total War. There's also II as well but it isn't finished so it may be missing some or actually have more idk. The game has lengthy texts from the historians of the era/archaeologists/modern historians for many landmarks, wonders, buildings, trade routes, peoples, classes of society, etc. While there is a max # of factions, they basically made all the rebel cities that are on the map inhabited by the tribes that lived there. So they are all like their own factions just unplayable but typically their units can be recruited as mercenaries or regional levies. They also patrol their territories, so they just aren't too aggressive. The most they will basically do is raid/attack inside borders. Tbh its probably the best researched mod I've seen.