It took the Russian Church a very long time to separate from the Patriarchy in Constantinople after all.
I think we can assume we have a stronger Axum and a strong Yemen (post-Himyarite) which is either part of Axum or a Christian vassal to it. That means Christians will have a large role in the Indian Ocean trade, and influence Kerala in India (and other coastal Indian regions, but I think Kerala would be the heartland of Christianity in India), and other areas like the Swahili Coast, Madagascar, the Maldives, and Indonesia. It's probably a fallacy to take OTL Islam in the Indian Ocean (which spread through similar factors of trade) and just switch it out for Christianity, but you'd have something similar. OTL Arabic script was used to write Malayalam, various Indonesian languages, and Malagasy. But inspired by their Oriental Orthodox brethen in faith, you might have Aramaic or Aramaic derived scripts replacing them (as Aramaic was used to write Malayalam OTL). Or more out there and less likely, Ge'ez--I think we could see Ge'ez being used to write Somali and some Bantu languages at least, although it will likely remain local to Africa. Madagascar will write their language in an Aramaic-derived script TTL, although it might be interesting if they use a Ge'ez derived script. I'd also love to see unique derivations of this, like the modern Dhivehi alphabet, spread widely and based on Aramaic or Ge'ez instead of Arabic.
IIRC, highland Ethiopians were more vulnerable to malaria than lowlanders. They also had an agricultural package less suitable for the lowlands than lowlander ethnic groups. And you have deserts to the north and east and swamps to the west which constrain their movements since their agriculture evolved in the highlands and was harder to adapt to other environments. This seems to have limited Ethiopian expansion.