I see.
So one possible early POD scenario would be an Ethiopian victory in the Abyssinian-Adal War thanks to Portuguese assistance, with the Ethiopians assisting the Portuguese against the Ajuran whereupon the Portuguese tenuously gain coastal areas like Mogadishu, etc for a time with the coast area being unstable until the arrival of the Italians, who while gaining Tigray Eritrea (and parts of Afar) and instilling an ATL Italian influenced Tigray / Agazian identity were ultimately pushed out of the rest of the Horn of Africa after the Battle of Adwa by the Ethiopians.
During that time a stronger Ethiopia is able to either redirect, butterfly away or assimilate the Oromo during their migration as well as attempt to assimilate other non-Geez groups in an Amharization / Geez-iation process, thereby potentially allowing Ethiopia to easily consolidate their gains upon pushing the Italians out in the Somali areas after the Battle of Adwa.
Or is there another approach involving a later POD as you already mentioned or at least a variation where the Ethiopians were able to assimilate the Oromo, Somali and others beforehand to more thoroughly demographically dominate the Horn of Africa up to Somalia / Somaliand?
Not sure how relations are between the Amhara and Tigrayan peoples in OTL, yet it would be fascinating to explore how they further diverge from each other (politically, economically, etc) along similar lines in ATL to OTL China and Japanese Taiwan / Taiwan, where one group is under colonial rule and another related group just manages to survive colonialism.