Except Azel!Arthur doesn't have the durability through skills or a holy weapon to take on mobs of enemies like Lex/Levin!Arthur can. Sending Azel!Arthur to fight a bunch of enemies is basically him getting hit by the first, counter-attacking and killing them, and then getting hit by the second and dying. It might take 3 hits to kill him, and he might dodge one or two, but neither will let him fight more than a few enemies at once reliably. And if you're just carting around a low HP Azel!Arthur to be a player phase nuke, I don't see him as being better than Midir!Lester (considering the latter doesn't have to wait to promote for a mount). I personally think Azel!Arthur is overrated and should be in Mid or something.
I think Midir!Lester can probably be Upper Mid; he gets inheritance and has pretty much exclusive access to the Hero and Killer Bows since Faval only cares about Ichival. I might be wrong but I think Hero Bow!Lester can kill the Dark Mages in Chapter 7 without getting countered, and they can actually do a lot of damage to most units. Then in Chapter 8 and 9 he can ORKO the ubiquitous wyverns, which is useful. He's not guaranteed to wreck the arena like Arthur but he faces the ranged arena, which is easier overall so he can get pretty far. I would put Julia in around the same place; she's not mounted but she has great EP combat with Resire and staff utility.