Hello there, in a series of continuing posts I plan on creating (or rather expanding and maintaining) a greater Aksumite/Abbysinian/Ethiopian Empire with the goal of it becoming something akin to that of Europes Habsburgs domains with colonies, kingdoms, duchies, and nation-states of their own, much like our timelines Habsburg realms I plan on having this empire come into conflict with the Ottoman Turks. My general plan for this is for it to go up until the modern-day starting in the day of Saint Kaleb of Aksum and ending roughly in the 21st century. (Although I will most likely go back and forth creating new stories or retooling old ones)
Before I start this I'd like to have a few suggestions in order to better align my thoughts with the expectations of those reading this tale.
Ethiopia? A Nation State? How? If Ethiopia should be united and a "Greater Ethiopia" it will have many other nationalities as nowadays Ethiopia. Unless you want your Ethiopia go on a genocide against the Afar and Oromo while forbidding the Tigray to speak their own language (Or reversed, the Tigray oppressing the Amhara because they are the rulers).
A Habsburg domain? Sure, if you mean Austria but with Colonies. Austria had many ethnicities with the same or slightly different beliefs (Catholics with Orthodox Christians and Protestants). There is one problem though: Austria is flat, it has no large remote areas as OTL Ethiopia has with many borders and being 75% size of Eastern Europe + the land that Ethiopia will also get (Somalia? Eritrea? Djibouti? Parts of Kenya, Sudan, South Sudan maybe?).
I don't think Ethiopia isn't able to modernize and be an even greater nation than they are now. They were a civilization longer than most of Africa and they have potential. But the situation is in no way comparable with the Habsburgs. You have a situation where Ethiopia has to start from the scratch. I think a comparison with the Ottomans is more likely as the Ottomans owned larger areas, had to recentralize the Empire for a long period to enforce reforms there while also being multi ethnic and multi religious Empire with a large part of the Empire being hard to access. Ethiopia is in the same situation.
My view... If Ethiopia controls the Amhara and Tigray lands by the start of the 1800s they could potentially consolidate power and make a move from that point on. If they control Eritrea for the sea access, Europeans can reach and trade with Ethiopia which is good for knowledge exchange. The war with the Ottomans is pretty much non priority for the Ottomans, as the Ottomans only owned the Coasts until the 1820s, with Ottoman Egypt reaching the Ethiopian Borders from Sudan in the 1820s. Which means, the Ottomans wouldn't put a lot of their resources on the Coast of Eritrea and let Egypt deal with it, making such victories easier for Ethiopia (The Ottomans lost Yemen many times to locals only to return much later).
Tl'dr: Ethiopia will be in no sense a nation state because it is too multi ethnic and will only succeed if they genocide every non-Tigray/non-Amhara population while forcing assimilation on the Amhara/Tigray (Depending on who rules). They would resemble Austria, Ottoman Empire and Russia in terms of demographics, the Ottoman Empire in terms of geography, population and decentralized situation.
By the way... unless your 5th century PoD does not affect much outside nowadays Ethiopia, the odds of the Ottomans or Habsburgs ever rising is zero due to butterflies. Which means you can fill it in any way you want for the 15th century. But I must warn you, such a large period can really demoralize your efforts. A shorter period is advised.