Arvis, Robin (M) or Cordelia for 5 star?

Just pick whichever one you will use the most.

I think Arvis would be a solid option. He brings something new to your line up with his strong debuffing and helps alleviate your mage shortage. He benefits from power creep with his legendary tome and exclusive B slot.

M!Robin would be good if you're struggling with archers but it looks like you're probably fine on that front.

Elincia, Cherche and Cordelia make up 3/4ths of a wicked Flyer Emblem team.

I’d promote Cordelia with that in mind.

Another mage would be nice, but you do already have pretty decent ones in each colour [Tharja, Delthea, Nino]. Standing up a strong Flyer Emblem team would be my personal priority. Self contained unit viable for a monthly orb and use in Squad/Arena Assault.

Honestly you don´t have a colorless 5* killer. I think you should promote Robin. As kagero, Takumi and Brave Lyn are still use a lot and can be a real threat.

While it may look tempting to go with cordelia with your current line up of rosters due to her IV's being perfect, unless you can get her a Darting Blow 3 or life and death 2 or 3 i'd wait since Cordelia's true strength will come then.

Arvis in my opinion is the best way to go. He would be a efficent debuffer on practically any team whether it is a physical one to debuff def/speed/attack or a magic one since you COULD get him a res ploy, although a high investment skill, or stick with his already build kit and be done right there.

Tl;Dr:
If you can't build your Cordelia rn then pick Arvis, he has almost everything he needs build in except fury/a special/ and a assist skill.