Who to promote?

Camus or Nino are great choices. Nino will help you counter the dominant blue units and Camus is a blue unit himself and he's pretty powerful thanks to his above average attack, speed and high def, all that combined with distant counter weapon and cavalry movement and access to cav buffs.

I recommend Sharena because she's featured unit every 3 seasons and gives u a satisfying score. My main team had her and Linde together for a long time and it worked pretty well.
Camus is also great, and Effie is a nice choice if you want to complete the monthly "beat tower stratum 10 with only armor units" quest for additional orbs - though you can do that with a 4* effie as well.
I don't think it's worth using 20k feathers to give Linde blarblade+, she performs perfectly well with her standard tome already and the higher mt. is pretty much irrelevant on a blade tome anyways.

I would say Nino is the best choice for offensive and Camus for somewhat tanky and defensive setup.
But since you have not ready for the horse emblem meme then you can save that choice for now.

I love my melee nuke Lucina, but red sword users are plentiful and you have bigger priorities.

Sharena would help you rank up in Arena, but you can probably get by with her at 4 stars until around tier 18ish if you have good enough units.

I'd go Nino or Camus based on what you have available for inheritence right now. Camus needs Fury 3 and Vantage 3 for his optimal build. Nino needs Death Blow, L&D 3, or Fury 3 along with Desperation 3.

If you're keeping Bridal Cordelia in your setup, you're limiting yourself to a maximum of 1 bonus unit and 2 other colors though. So since you have Linde already for a blue, I tend to lean Nino.