Who Should I Promote?

Your question could be answered with another 2 questions.

Do you have plans for each one? Which would make an immediate impact on a team as a unit used or as SI for an arena unit?

Answering those would tell you what to do. I don’t want to just call out a random name without seeing your plans, as that may be the wrong way for you to proceed. I want to help you, not waste your feathers. Lol

Maybe the l'arachel just to get it done with and clear up room in your barracks

Hmm...
I think Zephiel is outclassed by BK/Zelgius, but he has more Res if that's what you want.
Fallen Takumi is also outclassed by other archers, including non 5* exclusives like Setsuna.
Merric can be a great budget flier counter, but you probably don't need to promote him unless you really need a counter to fliers.
Your L'Arachels when merged together could make a great non-Reinhardt blue cavalry mage, but you already have 3 of them.
Mae is good if you don't have mages like Linde & Delthea.
Michalis can be extremely powerful. With weapon refinement he can either do insane burst damage with Ignis, or keep himself healthy with Noontime/Sol.
The rest (Henry, Oboro, Male Robin, Bartre, and Narcian) you should only build if you like the characters.
In the end, my vote goes to Michalis.

Depends on what you need really.

But In my personal experience don't waste feathers on Merric cause other green tome units can do the roles he is supposed to do better.

Any of the GHB are good. But with black knight appearing using Zephiel requires some investment to make him as good as the other sword armored units(but he's still good)and Takumi has a really niche weapon and IMO better defensively than normal takumi, And i think michalis is better than Narcian(Cause of the legendary weapon and defense)

Any of the mages are good to 5 star but the best IMO is Male Robin cause he's the easiest one to build, Just the Raven TA set is good for him,Henry and Mae are kinda hard to recommend(Henry has low attack and mae is supposed to be a fast mage but other mages can do it better) and L'Arachel is good on Horse emblem.

With Bartre he's a bad version of B!Ike(They kinda have the same stats but B!Ike has urvan)so if you want a bulky green unit go for bartre and same with oboro but blue.

Lachesis is a choice overall if you really need a god healer.