I agree with the majority of the list, although I haven't personally started my LTC run.
Things I would argue for:
Sylvain A- to A: Higher than Ferdinand because of his early access to the Lance of Ruin if you tell Rhea to fuck off. Also is one of the most versatile characters in the game, able to go Dark Knight for another physic user, or Wyrven Lord for just more everything. Also IMMEDIATELY available no matter the route if you play F!Byleth.
Dimitri from S to S: I want to reinforce that Dimitri belongs in S tier. Dimitri is just really optimized. He has the best growths in the game and snowballs early, preferably going through the bow-knight path. Admittedly while flight is a hard perk to measure, his route has access to Kingdom Knights/Gautier Knights/his personal battalion which is like +8might and +15 crit.
Seteth: B to A- or A: I know, I know, he comes late. But when he does arrive, he has literally everything going for him. Strengths in authority, axes, lances, flying, with good bases and growths in the right class. This dude literally has no weakness. Outside of the lords I would bench anyone else for him.
Dedue: C to D+ I hear that his viability is coming from smite? Idk but the dude has weakness in flight and horses, and is unavailable for a large portion of the endgame.
Ignatz: B- to C : I read the discussion you made earlier about breakpoints, and maybe your right, but his lack of access to any crest or DEADEYE(which is among the best combat arts in the game) and a strength in horses makes him in my opinion, C tier. Yes you can make him a dancer, but you can make ANYONE dancer. This goes more towards how good dancers are, rather than how good Ignatz is.