What is considered Ethiopia was the northern highland and it hasnt extended to the nearest coast since the fall of axum in the 700's.
The coastline of axum , modern day eritrea had its own states that were independent from any interior highland state.
So for ethiopia to get a coastline they would have to form a unified state in the northern highlands and expand north and conquer modern day Eritrea which was called midri bahri, they would have had to deal with the formidable beja cavalry and afar tribesmen.
Now to invade into Somali held territories they would have to go through several muslim sultanates . This they did manage to do under made seyon in the 1340’s.
The Ethiopians did not have much difficulty subduing states that fought like them like the several central highland and lowland muslim states, but had a challenge from Adal that had a combined cavalry and infantry army and managed to put up an equal fight for over a century despite a 3 to 1 manpower disadvantage.
This manpower disadvantage will fail against Somali nomadic tribes due to the logistical problem of supplying troops in the vast grasslands, and the fact that even sub clans could field light cavalry in the thousands .
The whole point is moot because to reach Somali lands they would need to go through the oromo like they did between 1880 and 1900. The oromo gadaa system allows mobilization of massive amounts of manpower and they would have matched any Ethiopian army in skill and manpower.
Realistically the only way Ethiopia within its current borders could have been created was exactly how it was done , through access to firearms and easy access to them provided by sympathetic Europeans.
The Ethiopians conquered the Oromo between 1880 and 1900 and they did this with firearms , and genocide. After killing half the population of 5 million, through war, famine and disease.
They turned the remainder into serfs. A large percentage of those became forcefully assimilated and now consider themselves Ethiopians and even part of the dominant amhara ethnicity.
The loose definition of the amhara indentity means that more than half of them today are actually assimilated oromo.