Good 5*+10 Candidates?

Cherche is probably the best one to invest because she is so common and easy to use. She performs well if you stick with her default weapon to deal with Armor units, or if you decide to replace it with something like Slaying Axe or Brave Axe.

If you want something more unique, then Est might be good because she has high Atk and comes with a Heavy Spear, which can be refined as a Slaying Spear. (I will note that I haven't tried this one yet, but may plan on building her up in the future.) Armor units are pretty prevalent higher up in the Arena, so having one unit that can deal with them is nice. Despite all of the memes of her running Def Ploy, that is actually a really solid option for her because her Res is going to be higher than a lot of the units she will be facing, especially Armor.

You can also go with Caeda, who is especially good now with her Proficient weapon Wing Sword. Same effectiveness against Armor, but she also gains effectiveness against Cavalry. She also has really good speed so can take advantage of the built-in Flashing Blade if you refine her weapon.

Cherche is a good idea, but Cordelia is also an really good option, although she's a bit harder to find at 4* and up.

My personal recommendation would be to 5*+10 armored units, since that's how you jack up your arena score. My 5*+10 Effie is a monster.

That being said, if you have your heart set on fliers, my current project is a 5*+10 Caeda and let me tell you- she is incredibly fun. Assuming she has the right support, she can tear through just about anything barring units like Effie or Nowi.

-Effie
-Nowi
-A!Tiki
-Myrrh
-p much most dragon and armor units

The 5*+10 tier list for f2p would be something like this:
S+: Nowi, A!Tiki
S: F!Corrin, any Armor unit
A: Fae, Cherche, Cordelia, Raven, Soleil
B: Nino, Caeda, Donnel
Dragon rule, high BST, very good enemy phase units(which is necessary in higher arenas), works well with an expansive build (Aether, Breath, Double Rally)
Armors have very high BST and access to OP armor skills as well as easily accessable armor buffs.
Cherche, Cordelia, Raven and Soleil are the best offensive units in the 3-4* category and work well with a Galeforce build(more Arena points).
Raven would be tier S, but he requieres more investment than Cordelia and isn't accessable at 3* like Cherche, also he is an infantry unit, so no emblem buffs.
Nino is the only mage here, cause of the low BST, however she is so strong, that I would cut her some slack. Same with Horse units(e.g. tiny hands man, Eliwood) Hana and Hinata would be B tier, if you don't need them for SI, however I would never recommend making 5*+10 Hinata your first project, you can never have enough of these furious boys around.

In terms of versatility, the manaketes (Nowi, A!Tiki, Fae, F!Corrin) will undeniably give you the best outcomes, but investing into them comes with a pretty hefty caveat that you will require Steady/Warding Breath & Aether to allow them to function to their full potential.

Armor units (ideally 175 BST ones like F!Hardin, but Effie and Sheena for budget purposes) are are a necessity if you are planning on aiming for higher arena tiers, but these units will also require a significant level of investment in terms of skill inheritance as well.

IMO, other units like Cherche and Cordelia are good projects to work on at some point but unfortunately they aren't as versatile compared to what dragons/armor units can offer.

Go with your favorites. A + 10 five star takes a couple of months and If you are not enough motivate with the unit you would probably abandon the project.

Any 3/4 star with the right skills will be really strong as a 5* + 10.

I have found Effie to be one of the best units in the game. Nowi is great but less points even with aether and you wont see much brave lyn in higher arena (advantage vs Sacae) and falchions can be a problem. +atk Effie is often beter as she can just oneshot any mage, red unit, or low def blues, ive even one shot some +10 nowi. I see a lot of berkut lance Effie but its not that great really.

She simply needs distant counter, Slaying lance and glimmer. She gets atacked, she couters with a 63 atk base glimmer, enemy dies. With a hone armor her kill spread is astronomical! Fortify armor is enough to survive two encounters easily. Wary fighter is still an increadible skill only lance-breaker or fully buffed Rein can one round her! Armor march is high scoring and even +2 BK gives good score to pair w her or build a sheena. Otherwise boots help but she is also a perfect panic ploy user, one of th best arena seals.

Cherche is good but gains little from merges and still basically a narrow melee glass cannon...