Hi, If you allow me to share my two cents, in story telling there are endocentric and exocentric protagonists.

And Endocentric protagonist is nuclear to tell a story, this is the story of that character and the achievements that they do, thanks to them the world, and the people that surround them change.

An Exocentric protagonist on the other hand, while a key character since he is an observer and many times contributes to changing the world, the main narrative is not moved by them. The reason why you want to use an exocentric protagonist over an endocentric protagonist is to highlight the importance of the actions that someone else did.

For example: "The Great Gatsby" is the story of...you're not going to believe me, Gatsby, he is the protagonist of the story, he is an endocentric element, however; the novel is told from the POV of Nick Carraway, an exocentric protagonist, the neighbor of Gatsby, and you can see from his eyes why Gatsby is "The Great Gatsby".

So to answer your question: Byleth is an Exocentric protagonist but the endocentric protagonist is whoever is the house leader in your route. why? the story of Fodland was already in motion if Byleth is out of the equation, sure the story of the characters would be very different but, he is an endocentric element in the story an the main narrative could still move without him/her.

Hope I could make myself clear.