I got a suggestion. You say the planet of Ram is very green and fertile. Not every planet needs to be turned into an industrial powerhouse. The empire requires food too. In fact, if it can easily man 2000 corvettes & destroyers with 15 people each, its probably consuming quite a few tonnes of food each day. So thinking as an Imperial Govenor: Why invest in factories when you can become a granary for the Empire?

The clans would be forced to pay a tithe to the humans living in the cities (let's act really xenophobic here and say they demand 90% of every harvest). The humans feel safe in their walled off cities and can act all high-and mighty about their civilian lives. While the hordes can hate them for practically taking their food.

Rebellion would be simple: first a few tribes in far away regions (like mountains or amazon-like rainforests) could seize sending a tithe. Patrols to demand the payement could get slaughtered and there you have it! Snowball effect. Other tribes rally to the cause, the conflict starts to escallate. Reïnforcements are called in from space (cause yk, the empire's food supply is at stake).

Again, just a mere suggestion.