Which to upgrade to 5*?

Depends on what you need, but I'd say Nino for the blade tome, Fae for Reinhardt tank, or Cherche if you lack physical attacker (requires a lot of SI help to be effective).

If your Spring Camilla is good, you might not want Nino or Fae, because you'll have 2 resistant hitting green units.
Cherche is your best option for strength, but imperfect IVs. You might want to consider Anna as well, with Life and Death/Fury and Desperation. Since you get her as a 2* she'll have plenty of SP to spend and is therefore quite easy to perfect.

I would go with Fae, who is pretty underrated, thats not a bad stat spread but he won't be great in PvP

Personally I would go with michalis, who is basically a slightly slower tankier Minerva, Luna and his personal weapon are great, iotes shield is cheaper, he's a flier, so reposition is amazing, he comes with threaten defense and utilities quick reposte very well.

If you want a reference to one here's mine: https://fireemblem.gamepress.gg/user-hero/suavegoose260/michalis

If I had to pick a second one it would be nino because blade tomes OP.