Worth keeping a -atk +res mia?

I would fodder her for Flashing Blade, that bane looks so crippling and I know I have several units that could really appreciate the skill.

Fodder for flashing blade. -Atk is unusable on pretty much every unit in the game

Give her fury desp

Or desp and spd +3 seal

Her offensive statline with her weapon is still basically 36 36 which passes the 35 35 benchmark

-Atk is not the end of the world. I have a -Atk+Def Mia and I use her all the time. I kept Flashing Blade, Vantage and gave her Moonbow, Threaten Spd 3 & Reposition. You could give her an Atk +3 sacred seal to fix her bane.

If she is your only one definitely keep her. She is perfectly usable and a lot of damage comes from her special proc anyways. If you have more than one, merge them.