Burgundy
, if that was the first time you ever got Sanaki, then I'd say keep her rather than using her for T-Adept. As a different red mage, especially one with with very high resistance, she can fulfill a different niche Sophia can't which in this case is straight up ignoring green and maybe red magic while Sophia is more of a "I can take hits from both physical and magic".
Still, it's your game, your units, and your decisions. What you do is up to you.
Not my idea, but if you really want to use him and make the most out of him, then give Zero/Niles: Fury 3, Quick Riposte 2/3, and whatever C-skill and Assist that floats your boat. I think I saw it before this time it was mentioned, but this is the most recent mention that caught my interest:
https://serenesforest.net/forums/in...ce-discussion/&do=findComment&comment=4801751.
As a neutral Zero/Niles, he has 14 wins, 0 losses, and 7 draws against all the mages in the game. Zero/Niles can't counter kill Cecilia, Henry, Merric, Odin, F!Robin, M!Robin, and Sophia. Some of those are for obvious reasons like Cecilia, Henry, and M!Robin have -raven tomes and Merric, Odin, F!Robin, and Sophia are bulky. With +Atk, Cecila, Odin, F!Robin, and Sophia are added as wins for Niles.
If you want a worst case scenario, if all the mages have Fury 3 and are attacking a neutral Zero/Niles, then only Leo and Reinhardt are added as draws. He leaves them with 1 HP. If they all have +Def which is kind of pointless for some of them like Linde, then only Tharja is added as a draw and with 1 HP as well. Basically, Zero/Niles has no losing matchups against mages if he's neutral or even with -Res. Anyone with attack or resistance buffs will just make him stronger.
There you go, a hard counter to hard check at worst to all mages in the game while also being a decent flyer killer. The only problem is this is a 5* Niles against 5* mages without any inherited skills other than the Fury example e.g. M!Robin with T-Adept. Still, Niles has a niche that no other archer can safely do. Rebecca can perform similarly if she has +Res, but not as well and it's more expensive since you'd have to give her a Killer Bow and Iceberg which Niles starts with. If Killer Dagger was a thing, I could see Felicia pulling off this build as well.