You can never rule a country by force.You either get killed,ousted,civil war.Solution is to strike deals with leaders of different regions,give them money.
Kenya is like ethiopia,Yoy have Kalenjin counties,GEMA counties,Luo Counties,Kisii counties,Luo counties...However in the case of regions where you have mutliple tribes ganging for power like Nairobi,It becomes chaos.Since devolution where power was distributed,Dont expect chaos,same should happen to ethiopia.