Minor Civs mean no challenge
That's also a common complain that barbarian spawned one city civs are not really a challenge. Maybe a little boost would help them by giving them free Golden Ages. GA length would depend on the era (of the leading civ). It would be 1 if it is spawned in the Classical, 2 in the Medieval, and so on.
I remember long time ago, when a barbarian city became minor civ, they usually raised another 1 or 2 barbarian cities, even if those were a bit far from their capital. They started with a settler among an army strong enough to conquer one or even 2 cities of their neighbor (and they usually did it). If you were not careful with that barbarian cities, you could be facing a 5-6 cities angry civ very early in the game. They were not only a challenge, they could put the entire game upside down. ( I can remember some minor civs leading the game).
Getting to this point needs, enough barbarian cities, having a real army when they appear, and, maybe the most important, the aim of using that army very quickly.