I can't really speak on Sherlock because I didn't use him. Dean, however... well I think you just need to see late-game Dean for yourself. He doesn't make the last few maps 'easy' per say, but he is quite a powerhouse. He's one of the few units who can actually fight Chaos if you don't train Faye.
When concerning Elbert, provoke is very situational but incredibly useful when such a situation arises. Any time a unit is on the verge of death or you want a boss to make the first move towards you, provoke is there to save the day.
Arthur is just all around a very consistent and reliable unit. He definitely falls off a bit lategame because his skills aren't too special, and his stats end up being pretty similar to others. However, like you said, his great bases make him a really reliable guy for the early and mid game.
I also benched Leon, so I can't speak too much on him either. I'm guessing it is just because they end up pretty radically different in terms of combat (like you pointed out). Volo can one round just about anything you throw him into, and he will also dodge for days. Also also also, it takes Leon a very long time to get Leon to that level while Volo is a show-stopper from the second you first meet him (shame that his character is so dreadfully boring).
Lastly, I also didn't use Izerna, so I can't say much there. My only guess is that healing is something that has a low bar... in other words, if you are capable of healing then you're good. After that everything depends on what else your character can do. This list has Owen placed higher than Izerna and I think it is simply because of how many other things Owen is capable of besides just healing.
ALSO I'm really glad you enjoyed the game 😄 I absolutely loved it and it is super nice to see others that also had a good time