Promotion question

A lot of them aren't worth promotion honestly. Reinhardt should only be upgraded with an attack boon for example. Cherche is best with +Atk/-Res and is fairly common on green.

Thajra can literally stay 4 stars as long as you have support for her and while her defense bane isn't the best, speed is the best Eirika boon you can get. The only one here I'd say to promote in a vacuum here is Valter. Adding Fury and Vantage to him makes him a solid blue melee duelist.

If you need more mages, Reinhardt is always a solid addition. Sure he doesn't have a +Atk IV, but just give him Death Blow 3 and he'll be good. +Atk Cherche is also a good promotion, except you have to invest a decent amount of feathers into building her. As for the rest, I've only really used Eirika, Alfonse, Sharena, and Anna in the past. Eirika is a good support, but most of the time sits back and lets a blade tome mage do most of the work. Alfonse, Sharena, and Anna don't have too much to offer, but can be useful if you need a bonus unit for the arena.

Reinhardt if you're willing to make it work (db lancebreaker), and while waiting for better IV (+atk)

tbh Reinhardt is good as long as it isn't -atk or something.

Also kinda bias but you want to promote Sharena if you don't have Ephraim or Blucina as buff and battle bot.

Reinhardt is a beast, and the +Res will help him against opposing Reinhardts. If you are using a horse emblem team he becomes a dominating force. Outside of horse emblem, he can still wreck with skills like Lancebreaker, blue tome breaker (against opposing Reinhardts).
I use Lancebreaker, and I usually quad strike the crap out of everything.

Enough about him though, and I do apologize for the lengthy post...
Sharena is a great support unit with a built in Fortify def skill and rally attack. Her 5* weapon gives her a free Threaten Atk 2 that makes her that much more efficient. Along with Eirika, they form a fantastic buffer support for blade tome users.

Valter is awesome with a built in Fury 2 for his Atk and speed. Combined with desperation and possibly Fury 3 (if you really want him to take 10 recoil damage every attack). Or you can go tank with him with skills like armored blow, renewal, or quick reposte. His HP is sky high (pun intended) so an effective albeit costly skill set would be earth (or fire) boost, renewal, and panic ploy. He generally has more HP than the ever present Horse Emblem so panic would trigger quite frequently.

Edit: Go with what you think will fill holes in your lineup or that you have good skill setups or inheritances for. A 5* unit that takes lots and lots of skill inheritance is sometimes less effective than one that takes little to no investment.