u/Ros80101

The route was supposed to be about Rhea/The Church

This is just incorrect all around. Silver is not a color associated with Rhea, but with Byleth. Byleth's face is there each time the war map is shown and the banner is the Crest of Flames. The literal first scene of the route is Byleth and Edelgard's falling out. Most of the route is Byleth focused on two things: Saving Rhea and stopping Edelgard(even if they don't want to harm her). Of course, Rhea is in this weird spot where you can't make the route about her because it misses the point yet having her barely be around the grow herself is also... unfortunate handling

Byleth has no motivation

This is also just incorrect. While yes, Byleth themselves has strange initial handling via not being fully voiced acted, the entire first half of the game builds Byleth up via them being a teacher. It allows them to grow in ways they never could as a mercenary, having purpose beyond being an isolated and infamous sellsword. And the way being them enjoying guiding others. Silver Snow is a bit of a tragedy of Byleth not being able to guide Edelgard. The route consistently has Byleth go something like "I don't want to harm Edelgard" or "Maybe we can walk the same path as Edelgard" which shows how hesitant they are regarding Edelgard, stuck between personal feelings and doing what they need to do

Something that has always intrigued me about Byleth and Leonie's dynamic is how both parties view Jeralt. One is his biological offspring who simply sees the man for who he is while the other is an apprentice who views him as a hero of sorts.

Leonie is a common village girl with a pretty decent life. Then in comes Jeralt helping out her village and she instantly latches onto him and starts to idolize him, wishing to be like him when older. Byleth, however, is his child. One who is there to witness his pros and his cons, from his leadership skills and combat prowess to his drinking problems and flawed parenting(even though Byleth very much appreciates that Jeralt is their father). At first glance, one would think them to simply be coworkers instead of father and child. Although with their unconventional dynamic for a father and child, it's hard to blame said assumptions.

Now what happens when you put these perspectives in the same room? You end up with Leonie, the one with hero worship, initially believing Byleth, Jeralt's flesh and blood, to not care much for him simply because they do not praise him the way Leonie does. I mean, when Leonie asks Byleth how they feel about Jeralt, Byleth explicitly says the following: "I respect him, of course.". This is an answer that Leonie does not seem too pleased to hear come from the child of one she looks up to.