Who to 5*?

This is basically just recommendations to five star for simple reasons. Take it as you like on what you are missing for teams or arena assaulting

Alfonse- Arena scoring

Eirika - awesome speed and support. Legendary weapon helps her role even better.

Athena - depending on IV's, she fast and bulky, which is super nice.

Masked Marth - Basically a neutral Lucina if you don't have one. Worth it, honestly.

Zephiel - He is, in my opinion, a favorite of mine and just all around awesome for armor teams. He's great even solo. Great legendary weapon with wonderful perk, and Wary Fighter to boot.

Sharena - Personal Opinion, but I think she is the best of Askr's starting squad due to speed

Effie- An absolute must as she is incredibly powerful. Good bulk, incredible offense with a brave lance, Wary Fighter, and can receive some good buffing.

Nowi- One of the best dragons around due to blue typing, Sword Breaker, and TA, which makes for a nasty threat for swords. Good BST for arena scoring too.

Anna - The second best for Askr's squad and is great for Arena scores. Green is good typing and she sports absolute amazing speed for a neutral IV unit. She is mega risky to play however.

Sheena- High resistance for an armor knight, let alone the best resistance for many heroes out there. She has low attack, but acts as a great wall.

Lloyd and Eliwood are honorable mentions just due to high resistance and acting as mage killers.

Well, from what I can see you have a lack of good blue tanks and not many colorless units.

I think I'd go with either Sharena, Effie, Nowi or Camus, depending on IV's, but M. Robin can be a pretty beastly tank as well. Also, if you have a SPD+ 3 star F. Corrin she's a great tank if you put fury 3 on her.

For colorless units check Kagero's IV's, she's pretty great. Or Leon and Klein.

Depends on IV's, what gap you need filled and how tanky your Mathilda is, IMO.